smudge,
as an ex Air Radar Fitter I do not recall any documentation that qualified me to work on the fighter a/c (Javelin, Hunter and Lightning) that I did minster to. But I can recall as a J/T (single upside down stripe) being issued with an 'oversigning chit'. This gave me the technical authority of a Cpl without the benefit of the rank OR the pay.
Once I became aircrew then the Cat Card (blue with black lettering as I recall) did define what I was qualified to do. And a certificate in my log book (signed by OC Eng Wing) authorised me to do Role changes.