Sorry for the late repsonse.

Short answer: both.

As explained in the description, the mod adds a range of the 13 & 18 speed E-F RTLO-9nn gearboxes covering differential ratios from 2.8:1 to 4.11:1 in approx. 0.2:1 increments.

For the stock vehicles (and some mods) it replaces the stock gearboxes with the corresponding one from this mod. The only real effect of this replacement is the gear name labelling with *possibly* a correction in a gear ratio or two (I don't recall off the top of my head if the stock boxes were 100% correct with all ratios).