To set a custom difficulty, you can either do so at the start of a new game, or you can do so between missions. This also means that you can change the difficulty over the course of the game if you wish. When you see the difficulty screen, you'll notice that there is now and Advanced tab. Clicking on it takes you to the custom difficulty options.
As of the 1.21 patch, the sliders automatically adjust to whatever difficulty was selected on the Basic tab (Sergeant, etc.) and become independent of the other tab once the Custom Difficulty checkbox has been checked. This allows the player to see what settings are used by each difficulty setting.
Currently there are no tooltips so I'll describe what each setting does. Some are straightforward and some are not, but I'll cover them all.
Custom Difficulty - You have to check this box in order for any of the sliders to override the difficulty setting selected on the Basic tab. However, the Advanced Options listed below will work either way.
Player Prestige - This slider affects how much prestige you earn. Note that if you set this to 0%, you will literally never earn a single prestige point (game breaker).
AI Prestige - This slider affects how much prestige the AI earns. As above, if you set this to 0% the AI will have no prestige to work with. I'm not sure if that would break the game, but certainly the AI would not be able to reinforce damaged units, etc.
Player Experience - This slider affects how much experience your units earn. At 0% your units will remain at 0-stars forever and you'll never be able to use overstrength.
AI Experience - This slider affects how much experience the AI's units earn.
Player Strength - This slider modifies the default strength of your units, as shown by the number over each unit on the map. Overstrength always goes up to five higher than what this is set to.
AI Strength - This slider modifies the default strength of the AI's units, as shown by the number over each unit on the map. Overstrength always goes up to five higher than what this is set to.
Number of turns - This slider modifies the number of turns you have available to complete each mission. The number in parenthesis is the actual number of turns added or subtracted.
AI Level - I wasn't able to find a detailed description of each setting, but Sergeant and Lieutenant difficulties use 0, Colonel uses 1, and the higher difficulties use 2. AI Level 2 is basically the best AI the developers have been able to create. On 0 the enemies tend to not move much in combat. They just hold their positions, for the most part. That doesn't count moving towards scripted objectives, of course. Level 1 sees enemies actively position themselves in combat to try and surround your units. AI units will move to support nearby AI units. AI air units seem much better at staying in groups. The AI's fighters will cover their bombers, whereas on level 0 they seem to each do their own thing. I haven't played on level 2 so I can't really say anything about it other than it's obviously going to be more challenging than level 1. Other players have said that level 2 AI is much more aggressive. In heavily scripted scenarios, the effect of AI Level is not as noticeable since the AI has to follow the script.
Game Rules - If you're new to the game, then this setting is probably irrelevant to you and you'll just stick with 1.20. If you've been playing since Panzer Corps came out, then you're probably used to the rules from 1.14 and earlier. The 1.20 rule set changes the following:
* Prestige soft cap
* Class-specific experience effects:
- AT gets +2 Hard Attack per 100 xp
Fighters get +2 Air Attack per 100 xp
AA guns get +2 Air Attack per 100 xp
* Overstrength is progressively more expensive with every point applied
* Initiative heroes are limited to +1 bonus
* Entrenchment gives a defense bonus of 2x the entrenchment level against ranged attacks
* Close terrain now gives +4 defense bonus to ground units vs. fighter and tactical bomber attack
* Overstrength is lost after upgrade
* Replacements come suppressed
* Train transportation takes 2 turns instead of 3
The prestige soft cap means that your units earn prestige as normal up to a certain amount. What this amount is depends on the mission, with subsequent missions in a campaign having a higher number. This slows down the rate at which players can potentially accumulate prestige and generally help keep things balanced over the course of a campaign. As a "soft" cap, you will still earn some prestige after reaching the cap, but only a small fraction of normal.
With the bonus damage per star (1 star = 100 xp), the smaller AT and AA guns become far more effective. Since most of the smaller versions can move further each turn, the damage bonus makes them worthy of additional consideration.
With the 1.14 rule set, overstrength cost the same amount of prestige per point, and the cost varied depending on the purchase cost of the unit. With the 1.20 rule set, the cost of raising overstrength from 12 to 13 is far higher than the cost of raising it from 10 to 11. In addition, upgrading between families returns a unit to 10 strength if it was overstrength. Upgrading within the same series/family doesn't reset the unit's strength. This means that with 1.20 you want to upgrade and then add overstrength, whereas with 1.14 you would want to add overstrength before upgrading.
Combat Random (Chance):
Normal - There is a chance that the prediction is considerably off and you are either lucky or unlucky. The possible deviation could be as high as 100%.
Limited - There is a chance that the prediction is somewhat off, but not by more than 20%.
None - Everything happens as predicted. Random chance only occurs with Rugged Defense.
Reform Units - This was a cheat that has now been given a place in the UI. If one of your units is destroyed, it will return to your unit reserve with zero strength at the begin of the following scenario. You can now reform it back to to it's basic strength by paying the costs for the unit to make it available for deployment again. This will preserve the unit's heroes and history list, but it will still lose all of its experience. If you chose not to reform the unit at the beginning of the next scenario you can keep it in your reserve list as long as you like.
Difficulty Presets
So that's all good information, but it would really help if we had something to relate it to. So now I'm going to write the settings that are used for each of the basic difficulty settings. This information comes straight from the diff.pzdat file in the Panzer Corps/Data folder. It's also visible within the game as of 1.21.
Sergeant
Player Prestige: 200%
Player Experience: 200%
Player Strength: 0
AI Prestige: 50%
AI Experience: 50%
AI Strength: -5
Turns: +5
AI Level: 0
Lieutenant
Player Prestige: 150%
Player Experience: 100%
Player Strength: 0
AI Prestige: 80%
AI Experience: 80%
AI Strength: 0
Turns: 0
AI Level: 0
Colonel
Player Prestige: 100%
Player Experience: 100%
Player Strength: 0
AI Prestige: 100%
AI Experience: 100%
AI Strength: 0
Turns: 0
AI Level: 1
General
Player Prestige: 100%
Player Experience: 100%
Player Strength: 0
AI Prestige: 150%
AI Experience: 100%
AI Strength: 0
Turns: 0
AI Level: 2
Field Marshal
Player Prestige: 100%
Player Experience: 50%
Player Strength: 0
AI Prestige: 150%
AI Experience: 100%
AI Strength: 0
Turns: 0
AI Level: 2
I thank the developers for adding these custom difficulty settings. Now for example, if a player finds Sergeant too easy/simple and Lieutenant too much of a jump, one can do something like use the Sergeant settings with an AI Level of 1, or whatever the player wants. Maybe you want a Lieutenant level of difficulty, but you want to change the way the difficulty is determined. Colonel settings with an AI Strength of -3 or -4 could do that for you. You can even change the difficulty settings between missions, so if you get stuck on one, you can lower the settings for that mission and turn them back up again afterwards.
Unlockable Difficulty Presets
Winning the last mission of any of the main campaigns (Wermacht, Afrika Corps, Allied Corps, or Soviet Corps, but not the Grand Campaign add-on) on Field Marshal will unlock the following difficulty levels, except for Ultimate. Unlocking Ultimate appears to require winning the last mission on Rommel, but that is not confirmed. If anyone knows the requirements for unlocking Ultimate, please let me know. Using easier difficulty settings prior to the last mission is permissible. If you don't feel like doing that for whatever reason, then just use the custom difficulty to mimic the settings. Doing so may (untested) prevent you from gaining the following achievements: Father of Blitzkrieg (win on Guderian), Strategic Genius (win on Manstein), or Desert Fox (win on Rommel).
Guderian (Same as General, but with 5 fewer turns to complete each mission)
Player Prestige: 100%
Player Experience: 100%
Player Strength: 0
AI Prestige: 150%
AI Experience: 100%
AI Strength: 0
Turns: -5
AI Level: 2
Manstein (Enemies have 15 strength)
Player Prestige: 100%
Player Experience: 100%
Player Strength: 0
AI Prestige: 150%
AI Experience: 100%
AI Strength: +5
Turns: 0
AI Level: 2
Rommel
Player Prestige: 50%
Player Experience: 100%
Player Strength: 0
AI Prestige: 150%
AI Experience: 100%
AI Strength: 0
Turns: 0
AI Level: 2
Ultimate
Player Prestige: 50%
Player Experience: 50%
Player Strength: 0
AI Prestige: 150%
AI Experience: 100%
AI Strength: +5
Turns: -5
AI Level: 2