RDF Concepts currently says:

"IRIs in the RDF abstract syntax MUST be resolved per [RFC3986]". There are IRI scheme-specific rules that should also be followed.

Examples of scheme-specific rules:

HTTP RFC 7230 -- non-empty authority (host), no default ports.

DIDs and URNs RFC 8141 each have a sub-grammar.

RFD Concepts can't enumerate all possible such rules but ti can mention they exists and like to HTTP/RFC7230.