Cos, I am not a metallurgist, so I cannot vouch for the authority of this site, but it is a starting point for US rules. https://www.marlinwire.com/blog/food-safe-metals-for-sheet-metal-wire-forms. They list a couple of grades of steel, namely Grade 304 and Grade 316 stainless that are approved, and some general guides as to what metals are inspected for (FDA):
It must not allow the migration of “deleterious substances or impart colors, odors, or tastes” to food
Be “durable, corrosion-resistant, and nonabsorbent”
Possess sufficient “weight and thickness to withstand repeated
Be “finished to have a smooth, easily cleanable surface”
Have resistance to “pitting chipping crazing, scratching, scoring,
distortion, and decomposition”
In general though, stock metals would be rated for reactivity, finish, a number of factors, and are assigned industry names or titles, and by grade those are declared food grade or not. Non-food grade metals can be treated as food grade for storage by putting a protective coating, but that would not make them food grade for cooking, only for storage. Cast Iron tends to be the opposite, approved for cooking, but not for storage as it is reactive to acids and porous.
Plastics are the same way, some are way too porous and absorbing to be used for food. We tend to assume that if it is being sold as a kitchen item it is food safe, and usually that is likely true. I only think it is an issue if we are attempting to adapt something which was not manufactured for food use, though there have been far too many instances of imported items into the US and EU that have blatantly violated manufacturing rules and skipped merrily past inspections making many of us edgy of certain countries of origin.
Edit from material provided from the NSF Food Equipment Materials Standard
The attached document from the NSF provides clear criteria and test methodology for features like 'cleanability'(section 5) 'corrosion resistance' (6) and then in section 7 lays out requirements for materials.
7.1 Stainless steel
7.1.1 Stainless steel used in food equipment shall be of a type in the AISI 200 series, AISI 300 series, or AISI 400 series .
7.1.2 When used in a food zone, stainless steel shall have a minimum chromium content of 16%. Stainless steel with a chromium content of
less than 16% may be used for cutlery, blades, and similar
applications requiring a sharp edge, provided the alloy has been
hardened or tempered by an appropriate post-weld heat treatment
7.2 Aluminum Alloy ...
7.3 Copper and Copper Alloys ... and it goes on.
While the term 'food grade' is never used this certainly looks like a standard that would be 'commonly referred to' as 'food grade'