How many ways do we have to set fields mandatory with material master

Hi Gurus,

Probably, this topic looks like a question, not a discussion.

While I am checking a question about fields mandatory, I found some fields are 'OPTION' in 'Assign Fields Selection Group', but those still are mandatory fields. For instance, fields 'MARA-MEINS (Base Unit of Measure)'. At the begging, I assumed that fields should follow some hard rule of SAP default, but I found another field 'MARC-PRCTR(Profit Center)' can be set same attribute as field  'MARA-MEINS (Base Unit of Measure)'.

I am very interested how many ways can set fields to be mandatory? besides Fields selection group (T-code: OMS9)

BR

Simon