PeptideSynthNJ said:

They are two different exemptions from the same federal requirements and they buy different things.

No disagreement with PeptideSynthNJ. One condition attached. 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.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

PeptideSynthNJ said:

They are two different exemptions from the same federal requirements and they buy different things.

This is where I part company with the consensus forming above. 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.

Answering the narrow version, because the broad one does not have a single answer. 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.

PharmD_Rodriguez said:

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

Agreed, though "tolerable" needs defining. A dose you tolerate by eating almost nothing is not tolerated, it is being paid for somewhere else — usually in lean mass, sometimes in adherence three months later.