Hi
You need to look in transaction OMSR. The field will be assigned to a field selection group and then the field selection settings for that group will control when the field is required. This may be something controlled via material type, plant, industry sector or transaction.
You are looking at field MARC-MMSTA which in the system I am working on is assigned to field selection group 51. This may be different in your system.
Regards
Robyn