>>298966
To try to define it, "A part or scene of an overall sequence of events that follows up on something that already happened.", though most people more commonly refer to these as a "sequel".
As far as fat art goes, when they're done, it's usually of a person in the same or similar setting while also using the same angle and similar pose as the previous artwork, with the character made considerably fatter, usually at least one full size class bigger. It's often the best way to show how much a character has gained since the last time they were drawn, in case you ever wanted to see them side by side.
However the Aerith "continuation" Salt did only keeps a similar pose, yet the setting and angle are completely different and her gains have been kept to the bare minimum. It also doesn't reference what happened in the art that the continuation is based on at all to it further disconnects them, as well as the overall tone being different as well.
While he did it for a continuation poll, it fails to feel like one at all and is instead treated more like it's own one-off thing rather than being connected to what it's based on, and that's why I'd call it bad.