OBD-II code family
Software Incompatibility With Body Control Module (U0331–U0339)
2 codes describing the same fault on 2 enumerated slots. The letter is the index your manufacturer assigned — it is not a physical difference, and it does not carry across makes.
What these codes mean
These two codes describe the same fault on two separate body control module slots. The letters — A and B — are indexes the vehicle manufacturer assigned, not descriptions of position or function. There is no industry-standard table mapping them to physical components, so the wiring diagram for your exact year, make and model is the only authority on which unit your code refers to.
Because the letter is an index rather than a physical difference, the diagnosis is identical for both. Everything that follows — the causes, the tests, the order to work in — applies to either code. What changes is only which of the two units you are looking at once the service data has identified it.
The one inference worth drawing is about the vehicle rather than the fault. A second body control module only exists on vehicles built with two of them, so a code naming the 'B' slot tells you the system is duplicated on your vehicle in a way it is not on most. If the 'B' code appears on a vehicle you would not expect to carry a second unit, the most likely explanation is a scan tool matching a manufacturer-specific code against a generic definition list — confirm the reading before any part is ordered.
Which body control module is mine?
There is no generic answer, and any source that gives you one is guessing. The letter is assigned by the vehicle manufacturer, and the same letter refers to different physical components on different makes — sometimes on different model years of the same vehicle. The only authoritative source is the wiring diagram or service information for your exact year, make, model and body style.
A manufacturer-specific definition always outranks the generic one. If your make publishes its own meaning for this code, use that and ignore the generic description.