A TFI is an identity used to distinguish TBFs.
Because we normally have many users in a PSET, each TBF is assigned an identity (TFI), so that any data that is transmitted on that TBF contains this identity. This helps the MS/BSS to determine which TBF the data belongs to.
A USF is a flag which is used to signal an MS to send. Once again because we multiplex PDCHs for many MSs, the BSS has to find a way to tell the MS when to transmit its data. This is done by assigning the MS a flag( USF) on each PDCH. Whenever an MS reads its USF in an RLC/MAC block, the MS can send its data in the next uplink radio block.
The TFI and USF are important for normal TBF setup. If there are no free TFIs or USFs on a PDCH (or PSET), then no additional TBF can be set up on the PDCH(or PSET).
Subhash, I already explained USFLIMIT and TFILIMIT in my first post