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[pd]

Simple way to describe it, cause I know what it's like when you hear about includes the first time...

Imagine you write your INI code down in a file, mark all it's content and copy-and-paste it into your rulesmd.ini.
That's what includes do: copy a whole file's content and paste it in the file it's invoked in.
So if you put the [VehicleTypes] list and the vehicle sections into vehicles.ini and your rulesmd.ini includes that vehicles.ini file, the game will act as if [VehicleTypes] and the single sections were in rulesmd.ini.

Advantage: You can use this to keep stuff well ordered.
I know I'm going to.