Leasing Licenses are limited by the conditions of use (read contract). In addition, you will not be the exclusive owner of this beat, so it will continue to be sold in my catalog. The Leasing Licenses are Basic, Standard, Premium and Unlimited.
The Exclusive Licenses have an indefinite duration and an unlimited use by the artist. Only you will be the owner of that Beat and all the rights to the master of the derivative work (song) will belong to you. If you want to buy the Exclusive License of a Beat but that option does not appear, it is because the Beat has already been sold with a Leasing License and therefore, it is no longer exclusive.