The If-Schedule-Tag-Match request header field is used with a method to make it conditional. Clients can set this header to the value returned in the Schedule-Tag response header, or the CALDAV:schedule- tag property, of a scheduling object resource previously retrieved from the server to avoid overwriting "consequential" changes to the scheduling object resource.
All scheduling object resources MUST support the If-Schedule-Tag- Match header.
If-Schedule-Tag-Match = "If-Schedule-Tag-Match" ":" opaque-tag ; "opaque-tag" is defined in Section 3.11 of [RFC2616].
Example
If-Schedule-Tag-Match: "12ab34-cd56ef"