Dr.SurgeonPGH said:

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

No disagreement with Dr.SurgeonPGH. 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.

That is the short version; the long version is somebody else's post.

Dr.SurgeonPGH 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.

Short answer first, then the reasoning. 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.

Dr.KarenChen said:

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

Agreed, and one detail people underrate: an irreversible payment removes your only leverage in a dispute, so it should be the last step rather than the first.