How does one become an Affiliate Professor?

I've noticed that several faculty members (both in my institution and beyond) are affiliate professors in some other institutions (beside being a professor in their own institution, of course). What caught my attention is that the vast majority of them wasn't in any way affiliated with the institution prior to holding the affiliate professor title (the source are their biographies). Afterwards they naturally collaborate on a regular basis.

Example: The professor did their BSc and MSc at my university X, went to university A in the US for a PhD, returned to my institution X (where they are now) and became assistant professor but in the same year became an Affiliate Professor at university B.

So, how does one become an Affiliate Professor?