The distinction that governs most of this is between a preparation made for an identified patient against a prescription and a preparation made in bulk for office stock, and the two sit under different statutory provisions with different testing obligations.
The salt-form point: the statutory pathway for compounding a copy of an approved drug during a shortage applies to the same active moiety as the approved product. A preparation described as a salt form — "semaglutide sodium", "semaglutide acetate" — is describing a different chemical entity from the approved base, and the description is usually there to construct an argument that it is not a copy. Whatever the legal merits, it means what is in the vial is not what was studied.
Twelve-month cost modelling, laid out: take the monthly product cost, add consultation or subscription fees, add laboratory monitoring at your chosen interval, add shipping, and then adjust the product cost for actual delivered content and dead-space loss. The route that looks cheapest per vial frequently is not cheapest per twelve months, because the fee structure and the monitoring dominate at lower product costs.
The statutory basis for the 503A/503B distinction is sections 503A and 503B of the US Federal Food, Drug, and Cosmetic Act as amended by the Drug Quality and Security Act of 2013, and the FDA’s guidance documents on each are the authoritative description of what is permitted.
Keep every document. The appeal you might need in six months is built from records you have to have kept now.