It's simply an extra where it didn't affect the main storyline. For example one of your character get killed off in the main story, but in the 'what if' extra, your character is still alive and kicking. Then it's just how thing works differently than the main storyline when that characters is still living.
Or another example if the main love interest in the main story is A and B, but in the 'IF' scenario, the love interest is A and C.
As an extra, they can be short, but sometimes long 'IF' scenario can even be 30 chapters long (I knew because I read some where the 'IF' is long and a very enjoyable read).
Some writer will put the 'IF' scenario intermittently between mainline chapters and continue just like that. Some wrote the 'IF' scenario after the main story is over, and then they can go crazy with how many 'IF' chapters they want to make.
(This is supposed to be an answer for ordinary guy but for some reason I can't reply directly with my browser)