* You need to activate storage unit management for the storage types you have in your WM under this Storage Location.
* You need to ave a partner location (without HUM) assigned to be able to unpack/pack goods in WM.
* You need to get the number ranges the same for HU and SU.
* You need to assign external number to SU (pick SU number from HU number)
* You cannot have negative stock in your storage types with SU/HU (except for the interim ones)
* HU and SU are very handy in warehouse, they contain all the info about the quant(s) inside the SU/HU
* In SU/HU you have already batch, quantity, stock category etc.so if you scan the SU/HU you don't need to specify any of that info, it is taken from the quant(s)
* In process of putaway, you first need to pack your goods in HUs to ba able to create a putaway TO (in LT03)
* In internal warehouse processes it is easy to use HU/SU
* During picking, you can assign a HU to the delivery, then pick everything to that HU
* Many more, you can find more info on SU/HU here:
Handling Unit (HU) - Warehouse Structure - SAP Library
Storage Unit Management - Warehouse Management System (WMS) - SAP Library
To customize all that, you need o follow the basic HUM customizing steps:
SPRO IMG => Logistics - General => Handling Unit Management
And WM customizing of storage unit number range:
OLML (Logistics Execution) => Warehouse Management => Master Data => Define Number Ranges