This is the version of the explanation I wish somebody had given me, written down before I forget what confused me. It is about compounded supply, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
They are two different exemptions from the same federal requirements and they buy different things. A 503A pharmacy is regulated primarily by the state board, needs a patient-specific prescription, is exempt from CGMP, and may use a bulk substance that has a USP monograph, is a component of an approved drug, or appears on the 503A bulks list — three independent doorways. A 503B outsourcing facility registers with the FDA, is inspected on a risk basis, must comply with CGMP, may compound for office stock without a patient-specific prescription, and has one doorway to a permitted bulk substance: the 503B bulks list, or the drug shortage list.
The condition it depends on
Worth adding the genuine exception, because it is real and narrow: a change made for an identified patient where the prescriber determines it produces a significant clinical difference for that patient. A grid of fixed doses offered to everybody is not that, whatever the intake form says.
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. I have searched first, so if this is covered somewhere point me at it and I will read it.
— SkepticalSean · corrections welcome and will be edited into this post with credit