The 501 (Not Implemented) status code indicates that the server does not support the functionality required to fulfill the request. This status code is commonly used when the server does not recognize the request method and is not capable of supporting it for any resource.

A 501 response is appropriate when the server does not recognize the request method and cannot support it for any resource, whereas a 405 (Method Not Allowed) response indicates that the server does understand the request method but the specific resource does not support it.

For example, a server that doesn't support WebDAV extensions might return 501 for PROPFIND requests. The server may also return 501 for methods it understands but cannot implement, such as a server that understands PUT but cannot actually accept resource uploads.