These would be field tests? Sorry, I’m not a regular dog person, I guess.
First of all, you have two different types of tests for retrievers: Hunt Tests and Field Trials. The Field Trials are the top of the pops, really you have to have a first-class dog, send him to be trained and professionally handled, and spend a lot of money to be competitive in field trials. They are a fairly technical and rarified test of a dog's ability to "mark" - i.e. see where a shot bird falls - and get there on the straightest possible line and return on the same line no matter what. Tremendously long distances - 400 yards is pretty common - and very difficult terrain.
The dogs that are bred for field trials are the absolute top of the retriever world -- very keen, very fast, very hard-headed, and VERY expensive. Not your average goofy friendly Lab, they can be very sharp and a real handful to train.
The hunt test is intended to be more of a test of the actual hunting ability of a retriever, and to create situations that as much as possible duplicate things that might happen on an actual hunt. The dog is expected to mark - on the higher levels 2 or 3 birds fall at once, as though you were in a group shooting multiple ducks, and the dog is expected to retrieve all of them, usually in reverse order though not always. The entry level is pretty simple - the dog just does "singles" - one mark at a time, two on land and two over water. On the next level, the dog has to do a "double" - two birds fall at once - and a "blind" - be directed by whistle and hand signal to a bird that he did not see fall (again, just as though you were hunting in company and another hunter shot a bird while you and your dog were looking someplace else.) On the HRCH level, the dog is expected to "honor" another dog, that is, sit still and calm while a bird is shot right under his nose and let the other dog get it, as well as ignore a "diversion bird" - a bird that is hand thrown right in front of him as he returns with a retrieve - and so forth. The distances are reasonable (120 yards or so for a mark and 75 yards for a blind is about as much as the dogs will be asked to do) and the terrain is what you might meet in the field -- fields, open water, the occasional "stick pond" i.e. flooded timber or a flooded cornfield.
To complicate matters, there are two associations that do hunt tests. The Hunting Retriever Club, which is part of the United Kennel Club, pioneered the hunt tests, and then the AKC got on the bandwagon because the hunt tests are much more popular than the field trials. In HRC, the levels are Started (SHR), Seasoned (HR) and Finished (HRCH). A dog that has made its HRCH has done some fancy retrieving under difficult conditions. If your dog gets a certain number of qualifying points you can go to the Grand Hunting Retriever Championship, which is held twice a year at various locations in the country. Our local UKC club just finished sponsoring the Grand last weekend. That is a big, big deal -- kind of puts our club on the map.
In AKC, the levels are Junior Hunter (JH), Senior Hunter (SH) and Master Hunter (MH). The received wisdom is that SHR is easier than JH, and my personal experience bears that out. My older dog is well trained enough that she could pass a Seasoned test now, I took her on a lark to an AKC test and ran her in JH. She handled it with aplomb, but she has a LOT of experience and this is supposed to be a novice dog event but it wasn't. Out of 32 dogs that started on Saturday, only 19 went on to the water test, and quite a number flunked the water.
Friend of mine says that the UKC judges WANT your dog to pass, while the AKC judges delight in flunking you.