503A refers to traditional compounding pharmacies operating under state board oversight and compounding against a prescription for a named patient.
503B outsourcing facilities register with the FDA, comply with current good manufacturing practice requirements, and may produce batches without patient-specific prescriptions.
Which is appropriate depends on the therapy and volume. Many telehealth brands end up using both, which is a routing question rather than a contracting one when the network is already integrated.
The distinction affects what can be prepared, in what quantity, and under what oversight. It also affects how a brand should describe its medications to patients and to reviewers.
503A vs 503B, in practice
It depends on the formulation and the regulatory conditions at the time. PharmaBro's network includes both, and routing selects the appropriate facility per SKU and state rather than locking you to one category.