The PATCH applies partial modifications to a resource.

PATCH is somewhat analogous to the "update" concept found in CRUD (in general, HTTP is different than CRUD, and the two should not be confused).

A PATCH request is considered a set of instructions on how to modify a resource. Contrast this with PUT; which is a complete representation of a resource.

A PATCH is not necessarily idempotent, although it can be. Contrast this with PUT; which is always idempotent. The word "idempotent" means that any number of repeated, identical requests will leave the resource in the same state. For example if an auto-incrementing counter field is an integral part of the resource, then a PUT will naturally overwrite it (since it overwrites everything), but not necessarily so for PATCH.

PATCH (like POST) may have side-effects on other resources.

To find out whether a server supports PATCH, a server can advertise its support by adding it to the list in the Allow or Access-Control-Allow-Methods (for CORS) response headers.

Another (implicit) indication that PATCH is allowed, is the presence of the Accept-Patch header, which specifies the patch document formats accepted by the server.