Defines.txt is a file in the Common folder and defines most of the constants that usually are hardcoded. This means that you have the option to change these values as well. We will not go into a full discussion on all the values here yet, but will list a few examples:
This is the amount by which stagnation is reduced by every merchant sent to a center of trade. It's a one-time effect, applied on arrival, regardless of whether the merchant is successful at competing. The value here is identical to the one used in-game, so the default -0.01 means every merchant reduces stagnation by exactly 0.01. So it takes 10 merchants to get the lowest visible change of 0.1. Greater negative values can be used to combat stagnation at CoTs that get competition. CoTs that are accessible by 4 or less nations (3 or less after monopolies are unlocked) do not get competition, as there are enough slots available for everyone to send the maximum number of merchants without competing.
This is the stagnation value at which a center of trade gets automatically closed. The value here is identical to the one visible in-game. Higher values can be used to keep stagnating CoTs operational for longer, and lower values will weed them out more quickly.
This defines the garrison size added for each fort level. If you wish to play without forts, you could try setting this to 0.
1.0 #MDEF_INFANTRY_SPEED_, 1.0 #_MDEF_CAVALRY_SPEED_, 0.75 #_MDEF_ARTILLERY_SPEED_,
Here we have the movement speed for the three main unit classes. You can change the ratio between them and make them faster/slower.
50 #_CDEF_CORE_LOSE_, //50 50 #_CDEF_CORE_GAIN_, //50
This defines the year limits on dynamic cores, how long you will keep a core before losing it and how long you must hold a province to gain a core.
This defines the Defender of faith cost. By default they are 1000.
The defines, unlike many other files in EU3, are relatively easy to understand. Although caution is still required, and it may be necessary to ask on the forums before change, a lot of the lines in the file can be easily deciphered.
You can usually just ignore the CDEF or DDEF and one must be careful not to get confused, weather the value is months, years or days.