I'm flatulanced!
The first number refers to the sweep, the shape/curvature of the edge. They are based upon a standard laid out a long time ago called "the London Pattern Book", the equivalent list is sometimes called the "Sheffield List." #1 is flat. . . . . #9 is a deep U-shape. The numbers run up into the 40's.
The second number commonly refers to the width of the gouge in mm (12mm approx 1/2")
F means fishtail -the shank is narrower than the edge area.
In Pfeil tools, 'D' refers to a set with shorter shanks and smaller handles, but the sweeps are identical
So, a D8/7 is identical to the full size 8/7.
Then for tight spaces, the shanks are bent = 'bent', 'front bent', 'spoon bent,' 'tracery bent. . . , all with unique LPB numbers.
Brands: concensus is that Pfeil is about the best. Ashley Iles & Henry Talor (UK) are up there, too.
You can find diagrams of the LPB in those company websites. The Pfeil logo is a an arrow (duh) on the steel shank and SWISS MADE on the very durable handle.
Size: sort of related to the size of the carving work to be done.
Big work (12"+) and big waste = 9/15 is a very strong U-shape sweep and it digs quite a hole.
I work a 5/35 with a 30oz lead core mallet after or instead of the 9/15. You can really get down to business in a hurry. Hard to push a big gouge and get much done, there's much better control with a big mallet. You can always choke up on the head for the tap-tap-tap thing.
I have a 2/30 but that's kind of the end game.
Smaller work: I still recommend a 5 sweep, 5/15 or so. I have a 5F/14 that can do a lot. With a 12oz Shopfox carver's mallet, nice combo.
Good, big gouges are not cheap. In use at 20 degrees, they take some tender loving care in edge management to keep them tuned up.
Now, for rough out when you don't have a big bandsaw. Make fret cuts and bash off the blocks.
Use a Stubai carver's adze (approx $140) for rough chopping/forming at the start. Use a Kestrel Tools elbow adze, they come in two sizes. . . you can haft your own blade. I have the Baby Sitka, it really is a "bandsaw on a stick."
Hope that helps. Any more Q, just ask.