Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. It is about compounded supply, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists. For 503B the shortage clause was the only route to these molecules, so that route shut completely. A 503A pharmacy can still argue a doorway via "component of an approved drug" — but only for the substance in the form present in the approved product, which is exactly where the base-versus-salt argument lives, and it does nothing about the copy restriction, which came back into force on resolution.
The condition it depends on
The enforcement dates were staggered by category — 503A first, 503B a few weeks later — because outsourcing facilities have manufactured inventory and clinic contracts to unwind while a 503A makes to order.
The practical version
Two things anyone can check: a state licence number for a 503A, and an FDA outsourcing-facility registration for a 503B. Both are publicly searchable, and a pharmacy unwilling to give you either has answered the question.
What I am not sure about
What I actually want to know is what actually distinguishes 503A from 503B, in terms of what each may make and from what starting material. If the honest answer is that nobody knows, that is a useful answer and I would rather have it.
— SkepticalSean · corrections welcome and will be edited into this post with credit