The way the Grapevine will work: Alice could attest that she trusts Bob to make music recommendations (the action) in the category of rock n roll (the category). And she associates the attestation with a certainty: 99% certain bc she’s known Bob forever, or 5% certain bc her attestation is based on only a brief interaction.
These are lots of options but the developer can simplify the user experience if appropriate by setting some defaults, e.g. set certainty to 80% and hide it from the user. That is how my desktop proof of concept works, which nostr:npub1p2uwv7qme2u92y2qcpqqvafhkkqsxfrrnz8m79lm60v4005s7vuqnexr0s is in the process of refactoring as a web app.