Under the leg (UTL) is where you reach under your leg with one hand to make the catch.
Chair if where you reach behind both ankles while in a seated position.
Behind the Back (BTB) is where you reach around behind your back.
To practice these catches it helps to string them together in a drill. Give yourself a light toss close to your body and then make an under the leg. Then quickly move to the Chair and then the Behind the Back. Also, be sure to catch and throw with both hands. This will ensure that no matter what spin this disc has and where it is you will have a trick catch that you can use to finishing your series of tricks.