What does it mean?
The amp's Master belongs to the amp block; the rig's Master comes after the chain. Both differ from interface gain and headphone volume.
ExampleExpression on Volume Position before reverb preserves the tail; rig Master fades the whole result.
How the meaning changes between controls
Amplifier
The amp block's level before the next block.
Pedal rigs
The overall rig level after the chain, distinct from amp Master.