Most sporting breeds have split between show and field. Cockers are one of those breeds. Cockers originally were just small English spring spaniels, but then were split off as a completely separate breed.
Within cockers there are English and American breeds. Then there are the field cockers. Field cockers are the field line within cockers. Completely different dog. Field cockers are high strung like you've never seen in your life. They are crazy energy. If you can handle a field cocker, you can handle anything. Field cockers are not for the faint of heart.
I just AKC spaniel (flushing) hunt tests. I've judged a fair amount of field cockers. Fun to watch because of the energy. The tough part is they are so short, in high cover, the often can't see their marks land, so their noses must be excellent to find their game. But unfortunately if they flush a second bird, they will completely forget there was a first bird. Lots of field cockers are trained by pros because they are a unique hunting style and require a lot of work. They are generally very smart and very sweet. Their body type and style is completely different from the show lines. You will find them to have longer bodies, shorter fur, shorter ears, and less high domed skulls.
If you get a field cocker, make sure you work them regularly, they are not great pets if you don't. Also make sure you have a pro you work with, for all the additional help with training you may need.
Field cockers are not for the faint of heart.