Various files of the Module System reference each other and are importing variables, definitions or tuples declared in the referenced files. You can see those at the top of each file with a line like
from module_system_file_name import *
It is sometimes useful to know how these files are all interacting with each other which is why this overview here got created. It is showing you the interactions at the Native Module System, there might be little changes at other modules. Simply click on a file name and see at the headlines below which files are interacting with the selected file.
To remind you again, there are four types of .py files in the ModSys folder, each kind with an own purpose:
header_*files - Header files contain constants for the game. You can change them, but it's generally not recommended or needed. If you want to define yourself a new constant, you can do it in the appropriate module_* file. All available operations are documented in header_operations.py.
ID_*files - ID files contain the numerical indexes for everything in your mod. In MABL/MBScript/whatever, everything can be looked at as a list of stuff represented by an index, and ID files contain these indexes. ID files get re-created with every compile you do, you don't need to change them yourself. You can use ID files to check what indexes your stuff has - if you get game errors that report an index but not a name, for an example.
module_*files - Module files contain all the game stuff, and they are the ones that you will edit. They contain nearly everything - troops, items, scene props, scenes, etc. You are allowed to edit them. Backing up your ModSys folder from time to time is recommended.
process_*files - The process files are responsible for compiling your mod into the game's beloved numbers. Modification to these files seldom happens, and before you do anything, you have to be absolutely sure that you know what you're doing - but for a start, just don't touch them. The process files, unlike the other parts of the ModSys, are written in real Python.