It is very much clear in the help documents however let me explain with an example
scenario Goods Receipt from production
MIGO ---Stock in interim Storage type 901( At this stage it is not possible to generate Storage unit as 901 is Non SU managed)-----Create Transfer order from 901 to physical storage type (At this stage if the destination storage type is storage unit management then system will assign a storage unit number) ---As a process operator will handover the SU label to worker who will paste the label on Pallet.
with preplan (LT0A) approach you can create the Transfer order (SU Label) in advance before MIGO and handover the SU label to the worker, Upon MIGO system will update the stock into interim (901) however based on material / quantity worker can paste the SU label which is generated in advance using LT0A further movement of SU can be done using t-code LT09