Field help (ITMREFSRH) 

This field is only accessible when the return entry makes no reference to any delivery or to no supplier return.
It is used to:

SEEINFO The entry of the product is used to initialize the ‘Description’ and ‘Sales unit’ fields that are modifiable. The sales unit can be modified on the condition of being chosen from the list proposed that contains the sales and packaging units from the product record and the units from the product-customer record.