The 100 series is used for conveying informational messages to the client. It is very uncommon to see this series, but possible to send the information status like this:

• 100 Continue: This status code indicates that the server has received the request headers and is ready to accept the payload that the client wants to send. The client first makes a request to the server; if the server responds with 100 Continue in response header, the client then sends the request body for further processing. The server can choose to respond with 417 Expectation Failed status if it is not ready to accept the request.
• 101 Switching Protocols: This status code indicates that the server is fine for switching the protocol that the client is requesting.
• 102 Processing: This status code indicates that the server is processing the request and it may take a long time for this to complete. This prevents a client from being timed out when waiting for the response.
• 103 Early Hints: This status code is used to indicate that the server is likely to send a final informational response to the client. It is common for servers to send HTTP responses that contain links to external resources that need to be processed before consuming the actual response; for example, rendering an HTML file or an image by a web browser. Sending early hints helps to prepare the clients to process the response.