What is most relevant at category level?

A colleague came up with a question that got some people in our office intrigued.

What does 'most relevant' mean in terms of a product category?

This makes sense to me in the context of search, but when the user has gone directly to a product category, how does 'Most relevant' apply as a sorting mechanism?

I understand this may have many answers, but I am edging towards maybe it is related to either your previous searches or perhaps even browsing behaviour?

Look forward to the answers!