What first stage means in practice
The first stage connects to the cylinder valve and reduces cylinder pressure to an intermediate pressure. It supplies connected components through the appropriate hoses. It is a different component from the second stage used at the mouth.
An example to think through
When booking a service, a photograph of the mouthpiece may identify only the second stage. Record both model names so the technician can identify the complete set and its service requirements.
A common misunderstanding
A brand name alone may not identify the components in a regulator assembly.
Underlying concept: PADI — Regulator. The example is an original YouDive study prompt, not a reported dive.