The HTTP Allow header indicates which methods are supported for a specific resource.
The HTTP Allow response header is used by a server to inform the client as to which HTTP methods are supported for the resource in question. In particular, if the server returns HTTP error response 405 Method Not Allowed, it must include this HTTP header to list what HTTP methods are, in fact, valid.
If the server returns an empty Allow method, then no HTTP request methods can be used. This can sometimes occur when a server is busy and is unwilling to process HTTP requests for resources with less priority.
In this example, the server indicates that the HTTP GET and HTTP HEAD methods are allowed.
Allow: GET, HEAD
The Allow header is sent by the server to inform the client of what HTTP methods are currently available for the resource.