Is "datum" member needed?

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.

"datum": {
"oneOf": [
{ "$ref": "#/definitions/geodetic_reference_frame" },
{ "$ref": "#/definitions/vertical_reference_frame" },
{ "$ref": "#/definitions/dynamic_geodetic_reference_frame" },
{ "$ref": "#/definitions/dynamic_vertical_reference_frame" },
{ "$ref": "#/definitions/temporal_datum" },
{ "$ref": "#/definitions/parametric_datum" },
{ "$ref": "#/definitions/engineering_datum" }
]
},

"$comment": "[RL] This datum (member) is not needed if CRSs are constrained to have explicit sub-types of datum."

Start with the quoted datum definition in the issue and read the linked ProjJson v0-7 RL 2024-06-16.docx. Determine whether CRS subtypes already constrain the datum variants, then record the CRS SWG decision and update the specification if a change is agreed.