Photos snagged off the internet...
WFTF, which is ironically the "world" variety of field target, shot all over the world, less than 12fpe (some do sit more upright, somewhat of a generalization, but this is the typically the class utilizing the most shooting aids that you don't see in other classes of field target and in some cases nowhere else in the shooting sports, ie shooting jackets and shooting gloves, and giant butthooks)
View attachment 414175 Next is Open Class of AAFTA (America only, < 20fpe). Defining feature here is the shooting harness, basically a backpack without the bag, that also has loops of straps that go over the knees so that the entire body becomes triangles of support, think bridges and radio towers and other architecture that utilizes the strength of triangles. Somewhat of a dying class...
View attachment 414176 Finally, Hunter Class of AAFTA (America only < 20fpe). Some sort of seat, and the gun is supported by shooting sticks or a bipod. Easiest position to get into and out of. Given the average age of field target competitors in the US, this is the fastest growing class and usually has the largest number of competitors at local monthly matches, as well as the larger GPs.
View attachment 414177 That's the summary of classes of field target in the United States. As for Britain, I believe there are a couple flavors, one which you mention where they have to be touching the peg. I think that is also the class where minimal scope adjustments can be made, and is often shot from prone.
@BC can hopefully provide some insi ght there, as he's an American FT shooter that has also taken part in matches in the UK.