The number of the camera frameset an aligned frame comes from, in the same numbering as /api/raw/latest and /api/color/latest: it lets a tool compare Intel's align on PC2 with an align on the host on the SAME frameset. Existing fields unchanged (frame_id still counts responses); the field-set tests now say "the R01 fields plus frameset_seq only". Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>