Regular is where you can't intevene. Meaning it won't simulate quarter by quarter. If you do intervention you can let the CPU simulate like the first three quarters & then choose to play the fourth quarter.
I'm a fan of this feature especially for side Dynasties. I always have a main Dynasty that I play most or all of the games on in the space of a year (at least for the first season) but I like to run a few other ones where I might play a few whole games but mostly jump in the fourth quarter especially during the playoffs.