A response representation of a resource is also accompanied by an ETag (short for Entity Tag),  which is a value that identifies the specific version of that resource.  It's used by API platforms to detect conflicting attempts to update a resource. 

A call to update an existing resource may check to see that the current version of that resource is as expected.  This is done by providing an If-Match element in the request header, with the value of the ETag for that resource.  If that ETag matches the resource’s current ETag, then the update is performed; otherwise, the request results in an error.

If-Match processing can be bypassed by specifying an If-Match element in the request header with a value of *, or by omitting the If-Match header.