Game.pex used to contain the function unlocking Enderal's Steam achievements. In 2.0.12, I moved it to a dedicated script because users sometimes installed SKSE on top of Enderal, breaking achievements. Enderal no longer calls Game.pex for achievements, but some older mods may still do this, so I still ship it with UnlockAchievement just in case.
In other words, if you play vanilla - it doesn't matter, if you play with mods on Steam - use Game.pex from Enderal.
In other words, if you play vanilla - it doesn't matter, if you play with mods on Steam - use Game.pex from Enderal.