Field help (SLOSIH4) 

This field specifies the sub-lot to be issued. When the lot is selected, the sub-lot is immediately initialized.
Specifying it is not necessary in the following cases:

If the product is managed with serial numbers, only this number is necessary to determine the stock to issue.
If not, the lot, sub-lot and location (conditioned by the product management method) are needed.
For credit memos, this field specifies which sub-lot is involved in the receipt movement.