Any registry/issuer can apply by submitting its own DID. Applications are queued for manual review by the root (MEG Initiative). Approval issues a signed accreditation; until then your issuer is listed as not trusted. This is the gate that prevents anyone from self-declaring trust.
The basis field is where objective evidence goes (e.g. an existing ISO/IEC 42001 accreditation). Today the root reviews it manually; later this can gate accreditation automatically.
MEG Address Sandbox - reference implementation of MEG1 Annex 23 & 24. Self-attested sandbox - no legal weight.