PF bi-nat

I've never had occasion to use bi-nat before and I'm struggling a little to wrap my head around the concept.

The OpenBSD FAQ (https://www.openbsd.org/faq/pf/nat.html) gives the following example:

"pass on tl0 from $web_serv_int to any binat-to $web_serv_ext"

However I'm not clear on how this fits into the overall filtering strategy ? i.e. building logically on the example above, how do I say "only allow inbound bi-nat for ports 80 & 443".

The FAQ makes an obscure statement "TCP and UDP ports are never modified with binat-to rules as they are with nat rules.", which I'm guessing is where the answer lies. But I'm not clear what this means in context ?

Thanks !

Laura