The initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.
When the request contains an Expect header field that includes a 100 continue expectation, the 100 response indicates that the server wishes to receive the request payload body. The client ought to continue sending the request and discard the 100 response.
If the request did not contain an Expect header field containing the 100-continue expectation, the client can simply discard this interim response.
Expect RFC7231 Section 5.1.1
Source: RFC7231 Section 6.1.1
100 CODE REFERENCES
Rails HTTP Status Symbol
Go HTTP Status Constant
Symfony HTTP Status Constant
Python2 HTTP Status Constant
Python3+ HTTP Status Constant
Python3.5+ HTTP Status Constant