Scope of top level entities if adapted from the ProjJSON schema

NOTE: Markdown does not support highlighting in color, so bold/italic has been used to highlight the material being discussed. This issue is extracted from the document submitted by Roger Lott at https://github.com/opengeospatial/CRS-JSON-Encoding/blob/main/ProjJson%20v0-7%20RL%202024-06-16.docx for discussion at the CRS SWG meeting OGC held during the Montreal June 2024 TC meeting.

"oneOf": [
{ "$ref": "#/definitions/crs" },
{ "$ref": "#/definitions/datum" },
{ "$ref": "#/definitions/datum_ensemble" },
{ "$ref": "#/definitions/ellipsoid" },
{ "$ref": "#/definitions/prime_meridian" },
{ "$ref": "#/definitions/single_operation" },
{ "$ref": "#/definitions/concatenated_operation" },
{ "$ref": "#/definitions/coordinate_metadata" }
],

The ProjJSON schema includes some entities at the top level that are lower level entities in the CRS Abstract Specifications (ISO 19111 and OGC topic 2). Should scope be limited to 'top level' entities, i.e. CRSs and coordinate operations? If so, remove from this list the lower level entities: datum, datum_ensemble, ellipsoid and prime meridian. (These [and other] lower level entities will be needed as components of the top level entities so are not eliminated from the schema, just changing the schema to define only top level entities)."

Start with the linked ProjJSON v0-7 proposal and compare its oneOf list with the CRS Abstract Specifications (ISO 19111 and OGC Topic 2). Confirm whether datum, datum_ensemble, ellipsoid, and prime_meridian belong as top-level entities; done means the scope is agreed and the schema reflects that decision.