It has to do with the use function, which is really just syntactic sugar. One of the things it does is to call the static import method on the package you’re loading, passing it the parameters you gave on the use line to process them. The common behaviour of import is to export functions from the package it’s in into your current one, usually by calling upon Exporter. However, all of that is entirely conventional, and you can actually do whatever you want with those parameters – like using them as a names for configurations to choose between, as Herkum does.

The use docs have the details.

Makeshifts last the longest.