in other words,
TFI is a logical identifier… it is used to distinguish the owner of a block from another MS.
USF is a green light, sent in DL to the MS. Every time the MS gets its own USF in the DL block “header”, it can upload its own data in the next UL block.
USF is just a mechanism to multiplex several UL MS in one UL timeslot. This green light is dynamically distributed among all the UL users. Each block.