“Instrument” is just a generic term for a variety of bits of paper that do various things.
It could be a direction under a regulation, it could be an approval under a regulation, it could be a determination under a regulation, it could be an exemption from a regulation, it could be an airworthiness directive, it could be a permitted unserviceability ….
Some are disallowable; some aren't...
Best to read the specific instrument – it will say what it’s for.
But don't worry: They'll all go away under the new, simple 1998 regulations...