Academic logic

This one is in the rules and is assessed with judgement. The embassy verifies that your course is consistent with your academic background (MUR circular), and "consistent" is a judgement about your particular path.

What it is really about

Two things. Your qualification has to permit the course, which is the recognition question (V6). And the course has to make sense after what you have already done.

Three situations, and what I can honestly say about each

• A straight continuation. HSC in science to an engineering bachelor's, a finance bachelor's to a finance master's. Nothing needs explaining.
• A subject change. Allowed, and ordinary. Your university decides whether your background qualifies you (M5), and if it admitted you, a body of academics has already judged the fit. Your admission letter is evidence of that, and it is evidence an officer can see.
• A step that looks backwards. A master's holder applying for a bachelor's, or a second degree at the same level. This needs an explanation that holds up, and I do not know how the embassy weighs it.

On study gaps and modest grades

Which is the question most people actually mean: V23.

What I do not know

Where the line sits. No source defines how unrelated is too unrelated. Your admission is the strongest evidence you have that the fit is real.