When you mark a subfield mandatory in the MARC framework, that has the effect of making the tag mandatory as well, regardless what the "mandatory" flag on the tag is set to. I think that marking a subfield mandatory should have the effect of "mandatory if applicable," which is used commonly in the MARC standards but can't be represented at present in Koha's MARC bibliographic frameworks.
I agree with Jared. We should have the ability to define mandatory subfields in non mandatory fields.
I don't thinks is true anymore. I have checked the normal and the advanced editor and both test for 245$a nicely.