Yes, thats pretty much it. I probably should not have used the term "master" especially in relation to Render Layers as it might get confused with the "Master" toggle in the Render Layer node, which refers to something else. A better term is probably "parent", "parent & child".
But yes the idea is when one field is set to be a "child" of a field in another node then the child's inputs are entered not manually but derive from whatever is entered into the "parent" field.
Nuke used to have a way of doing this (it still may, i haven't used Nuke in ages
) with variables, similar to the way Terragen uses variables to name render outputs, and After Effects does the same thing between values in layers. With the Nuke set up there was a syntax where you'd identify the node and the field and the connection was set up.
