DeniseRN_TPA said:

Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists.

That is correct as far as it goes, and here is where it stops going. The shortage clause is the answer to the second question and it is a subtraction rather than an addition. Both exemptions forbid compounding something that is essentially a copy of a commercially available approved product. A product FDA has listed as in shortage is not treated as commercially available, so listing removed the objection that otherwise blocked compounding. It never created a permission; it withdrew a prohibition, which is why it evaporated the moment the supply fact changed.

DeniseRN_TPA said:

Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists.

Pushing back on DeniseRN_TPA here. A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to. It is a different legal universe with no pharmacy oversight, no patient relationship and no content guarantee, and conflating the two in these threads helps nobody.

Taking the question as asked, rather than the general version of it. 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.

Dr.MetabolicMD said:

The shortage clause is the answer to the second question and it is a subtraction rather than an addition.

Same experience, arrived at from the opposite direction. I had assumed I was the exception until I read this.