Which HLS playlist attribute marks spatial/immersive video, what values may it take, and what must the MP4 contain?

Apple's authoring spec requires CH-STEREO/PROJ-AIV for immersive video and states PROJ-AIV is defined outside core HLS; IETF draft-22 (1 May 2026) lists only PROJ-RECT/EQUI/HEQU/PRIM, so PROJ-AIV is absent from the draft value list. Curator qualifications: Client behaviour on unknown specifiers (draft says ignore tag and URI) and Apple example CH-MONO/PROJ-RECT I-frame entries not re-read line by line. Draft is an Internet-Draft (work in progress, expires 2 Nov 2026), not an RFC. Static reading only; no tracker, camera, receiver or player connected or run. Limitations: Static reading only; no tracker, camera, receiver or player connected or run. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://www.ietf.org/archive/id/draft-pantos-hls-rfc8216bis-22.txt",
      "note": "Apple HLS authoring spec: 'About spatial video' and rules 16.6-16.7; draft-pantos-hls-rfc8216bis-22 lines 2845-2868"
    },
    {
      "url": "https://developer.apple.com/documentation/http-live-streaming/hls-authoring-specification-for-apple-devices",
      "note": "Apple HLS authoring spec: 'About spatial video' and rules 16.6-16.7; draft-pantos-hls-rfc8216bis-22 lines 2845-2868"
    }
  ]
}