I'll go ahead and explain it.
Followup attacks are not necessarily the same as following up separate attacks in a combo. Rather, followup attack exist "within" a particular move. Let me provide and example with my main, El Fuerte.
His Habanero Dash (236+P) has him run forward, but in this run, there are several things he can do.
Backwards Habanero Dash (214+P) - Run in the opposite direction. You can actually dash dance with this. Makes people mad.
[COLLAPSE="Button Followups"]
Pressing these buttons in the Habanero Dash will give you these options.
LP - Stop in place
MP - A overhead bodyslam where you must block high.
HP - A jump in command grab that must be blocked low or evaded.
LK - Step back, used for feinting.
MK - Runs in with a kick which can combo into his super combo among other things.
HK - Slides in causing a knockdown.
From the Backwards Habanero Dash
LP - Stop in place
MP - A overhead bodyslam where you must block high.
HP - A jump in command grab that must be avoided as you can't block it high or low.
Any Kick Button - Jumps back a far distance.
[/COLLAPSE]
With these examples, these moves are only available from the dash. You will have exceptions where an attack is available as both a followup AND a standalone attack.
This kind of followup is used for mixups, making your opponent guess what you're gonna do next, especially on their wakeup. There are other types though, but this example gets the point across.