Game System files are specific to the rules for a gaming system, such as 3e, Unisystem, D6, etc. These files define what rules apply for that system, such as Hit Points, vs Vitality Points, Armor Class vs Damage Reduction, etc.
There are six files in a Game Mode, they are:
equipmentslots.lst
level.lst
load.lst
miscinfo.lst
rules.lst
statsandchecks.lst
*** Update 5.9.x
In the 5.9.x line the remaining system files being moved into the gameMode so they can be customized for any specific game. System files which have recently become gameMode files are:
bio/biosettings.lst
bio/locations.lst
bio/traits.lst
bonusstacks.lst
pointbuymethods_system.lst
sizeAdjustment.lst
tips.lst
unitset.lst
A new directory named "default" has been added to the gameModes directory which contains default versions of each of the newly integrated files. If a gameMode is loaded which does not contain one or more of these files then the files from the default directory or loaded in their place.