100 Continue
Meaning
This interim response indicates that the client should continue the request or ignore the response if the request is already finished.
When to Use
When the server has received the request headers and the client should proceed to send the request body.
Example
Client sends a large POST request with Expect: 100-continue, server responds with 100 Continue to proceed.
When Not to Use
Do not use if the request is already complete or if no request body needs to be sent.
Source
RFC 9110