What fit means in practice
Fit describes how equipment suits the person and the rest of the configuration. It includes comfort, access to controls and compatibility. A size label is a starting point for assessment, not the whole assessment.
An example to think through
For a rental request, describe what you have used and any recurring fit problems. During the fitting, ask the provider to resolve those concerns before treating the size reservation as complete. Record the model that worked for future reference.
A common misunderstanding
The same letter size across different products need not produce the same fit.
Underlying concept: PADI — Buoyancy Control Device (BCD). The example is an original YouDive study prompt, not a reported dive.