I believe this is a field to show the inventory received in R status. 

this field is only used (and accessible) for products not managed in stock. It is used to enter in purchase unit the receipt quantity that does not comply with quality standards. As there is no stock management, there is no stock status to identify quantity that are not conformed. This is the reason we have this dedicated field. 

This quantity can be used in the supplier returns (according to the PRTNQTY general parameter you can set this disputed quantity to be the quantity to be return) and in the supplier invoice control entry (according to the PIHQTYAQR general parameter impatcing the quantity to invoice).

If there is no definition on the on-line help, please report a bug (documentation bug) to your local support. Sometimes, the documentation exists but not in the current language.