Whether or not you have a "six pack" is entirely based on body fat.

It's a sign of skinniness, not health, and only womewhat fitness. It has nothing to do with how muscular you are: I always had one when I ran long distance seriously, which rips off upper body muscle.

— Ian Welsh (@iwelsh) May 18, 2020

Yep. You can kinda see my abs now, but if I wanted a six-pack I could have one in two weeks if I just did some fasting.

Having a six-pack is not any sign of fitness. It just means you have low bodyfat (for a man, usually 10% or below). Here’s the thing, though: I am still attempting to gain muscle, not look ripped shirtless. That means I don’t want really low bodyfat. That is inimical to my goals.