TBF reallocation is not a parameter, it is a procedure.
When a TBF is allocated and suddenly one of the PDCH timeslot is preempted for a TCH allocation, then you see the TBF must be reallocated : from 4 PDCH it is now reduced to only 3 PDCH.
This is a TBF reallocation.
Causes for reallocation are mostly :
1/ TCH preemption (= reduction of PDCH)
2/ Optimization (= increase of PDCH because the cell is less loaded now than before)
3/ change of bias : the DL TBF is less used than the UL TBF, then DL TBF is reduced and UL TBF is increased
There are some other causes for reallocation, ut they are very rare and complex to explain 🙂