That's the dial code to start an intercom call. For example, you dial *9100 if you want to make an intercom call to extension 100. If you need to make a call to that number from the 3CX Call Control API, you just use the MakeCall method and set one of the endpoints to "*9100".
Please note that the dial code is disabled by default. You might need to go to your 3CX Console > Settings > Dial Codes > and set the value *9 (or any other) for Paging.