SUBGROUP_DELIVERY_TIMEOUT (Property Type 0x06) is a Track Property. It is a varint. Its semantics are defined in Section 8.
§12.1.
SUBGROUP_DELIVERY_TIMEOUT
This is one section of the MoQT specification, rendered per-section for quick reference and citation. The authoritative text is draft-ietf-moq-transport-18 at the IETF.