This is to recover real index of surface in common surface sets.
In any times you define index based on your mod, this mean index `1` in two different mods it should convert to different real index like `2001` or `4001` (and this value will change when you swap mod load order).
First value initial value is surface index in given mod, second is mod id.
As one another details, this function some times do not add mod offsets, because some index are "common" and all mods share same surface, this is for base game asserts where every mod can override it or use in its own units.