What's the the difference the fat cap?And quit downloading pics of Franklin's Q 
Franklin doesn't know the first thing about a kamado. Or at least didn't, a few years ago, when he had his BBQ with Franklin show on PBS. He had an episode talking about grill styles, and talking about putting a whole split of wood into the firebox of a kamado, which is wrong on so many levels. He cooks with offset smokers... The "pure" way ;-)
I have recently picked up some customers in Texas, though, and I'm hoping to make a trip to Austin and sample his brisket from the source.
As for the difference, a "whole packer" is comprised of two cuts, the brisket "flat" and the brisket "point". They're separated by a vein of fat between them, but that's a completely different vein than the fat cap, which is on the opposite side of the flat. The point is a lot more fatty than the flat, and is more typically used for burnt ends or chopping, rather than slicing--although you can slice it.
Typically flats are used for pastrami or corned beef. They tend to run much more expensive on a per-pound basis than whole packers partly due to that usage, and partly also because to prepare a flat, you end up cutting away a lot of fat. When using in a BBQ setting, I prefer the whole packers because you get that fatty point, either for burnt ends or used elsewhere.
Someday soon I'm planning on giving pork belly burnt ends a try, though... That sounds like fun but I haven't done it yet.