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What 100 means
The initial part of the request was received; the client should continue sending the body.
Sent in response to an "Expect: 100-continue" header. It lets a client check whether the server will accept a large request body before actually sending it.
Informational (1xx)
Provisional responses that tell the client the request was received and processing continues.
Other 1xx codes
FAQ — 100
What does HTTP 100 mean?
100 Continue: The initial part of the request was received; the client should continue sending the body. Sent in response to an "Expect: 100-continue" header. It lets a client check whether the server will accept a large request body before actually sending it.
Which class does status code 100 belong to?
100 is a Informational (1xx) code. Provisional responses that tell the client the request was received and processing continues.
Where is the 100 code defined?
Status code 100 is defined in RFC 9110 and is part of the IANA HTTP Status Code Registry.