What is extreme multi-label classification(XMLC)

I tried solving a problem in recommendation systems using PyTorch, and i came this in an article, so ill like to how to use it and apply it to solve problems in recommendation systems?

Based on this post by PapersWithCode it seems that Amazon provided a library based on PyTorch called PECOS for this use case, so you might want to take a look at their implementation and how the large space of labels is taken care of.