Confused by SS Specs and Grades 2

What spec system are you working in? ASTM?
304 is a historical common name that doesn't mean much.
The chemistry is defined by the UNS number, S30400 in this case.
This has nothing to do with mechanical properties.
Then you go to ASTM and select a spec for your product form.
This is where properties and testing specifics are listed.
In that spec it will refer to the general specs that cover broader areas.
And it will list testing (mechanical, destructive, and NDT) that apply to products.
You usually end up needing 4-8 specs to fully document a given product.

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P.E. Metallurgy, consulting work welcomed

Some of our materials are per ASTM and some in SAE.
Some of our materials are custom like tabs or clips for which no ASTM or SAE spec seems to be applicable, so we're just calling them our as 304 SS.
From what I can tell the original grades such as '304' originated by a joint AISI / SAE effort.

All I know is P/A and Mc/I

They are not 'just 304".
They are made from some form of raw material and that material has applicable specifications.
The SAE "J" specs usually take the place of ASTM product specs but refer back to ASTM general and RM specs.
You need a class in working with ASTM specifications.
These are the legal basis for the orders and part acceptance.

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P.E. Metallurgy, consulting work welcomed