# scale_pos_weight effect in XGBClassifier

I can't find a satisfactory explanation about the effect of scale_pos_weight on an XGBClassifier.

It says everywhere to set it to Count of negatives / Count of positives, but then if there really is no other choice, why is it possible to modify it ? What does it do concretely and what would be the effect of setting a different value ?

XGB's documentation only says "Controls the balance of positive and negative weights" which is not very precise. And when I look it up in the code, it does not seem to be used anywhere (at least in .py files).