LPDAL, As others have mentioned, Big Front Seat is NOT a separate class of service nor a separate cabin. It is simply a seat with a price commensurate to how much room it has. Just like you pay more for an exit row seat with more legroom, or an aisle toward the front, than, say, a middle in the last row.

That is why it is not listed out as bookable from the start. You buy a coach ticket, and the seat may be purchased in advance at the appropriate part of the booking process. Additionally, one may get lucky and get this seat for free. On Spirit, you have the option of getting a randomly assigned seat at check-in. A colleague of mine got randomly placed into a Big Front Seat by the kiosk on an ATL-DFW flight (on a $270 last minute round-trip bought 24 hours out... comparable to $800 on DL/AA).

Zone 1 boarding is for those who purchase a carry-on bag, and those in Big Front Seat will get Zone 1 if they meet that qualifier, because again, it's all coach.