Processing

The server has received the request and is working on it, but no response is available yet.

102 Processing is a WebDAV interim response telling the client the server is still working, so don't treat the connection as dead. It dates from when long DAV operations, like copying a large folder tree, could run for minutes. The newer WebDAV spec dropped it for lack of real-world use, so outside legacy DAV servers you'll almost never see one.

What 102 looks like on the wire

MOVE /projects/site HTTP/1.1
Host: dav.example.com
Destination: /archive/site

HTTP/1.1 102 Processing

About 1xx Informational

The request was received and the process is continuing. These are interim responses you rarely handle directly.

Where it's defined

102 Processing is defined in RFC 2518 §10.1. MDN's 102 reference covers browser behavior and examples.