The boundaries are a _little bit_ vague of course. Traditional attacks transition from "data" (the buffer) to a reality where all control is handed to the attacker (ROP, traditional executable stack pivots, etc.)

The attacker controls what's executed, and stitches side-effects together to achieve the desired attack. This also starts with "data" but ends with "instructions of the attacker's choosing doing what the attacker wants".

In a data-flow attack (which _still_ has some characteristics of the former), the attacker does _not_ have the ability to pick-and-choose what gets executed, even in the final stages of their attack. They _still_ need to overflow the buffer, and put some other data in some other registers, but their attack _never_ gets to the point where they pick the next instruction.

Likely, at the end of the exploit, arbitrary code execution or privilege escalation has been unlocked. But the exploit chain itself doesn't go through a stage of arbitrary execution (either through ROP, or executable stack). That's how I like to distinguish anyway.