TIS-B is a service that broadcasts traffic information. This traffic information is gathered by using various surveillance sensors such as ADS-B and/or radar.
In case the surveillance source is a Mode A/C radar, and no ADS-B track can be correlated to the radar track, no 24 bit address is known for the aircraft. Now the TIS-B message will be filled with a 24 bit address that is created ad-hoc and temporarily assigned to that radar track. This is not a standard ICAO address (which is assigned through official procedures of the ICAO member states) but instead a (more or less random) number is used.
To clearly distinguish between the ICAO 24 bit addresses and the ad-hoc addresses (which may be accidentally the same) a different message indicator is used in TIS-B.
A similar scheme is used for ADS-B and ADS-R. In case an ADS-B transmitter cannot be assigned an ICAO 24 bit address (e.g. because the country only assigns addresses to aircraft and the transmitter is on a car or a fixed obstacle) an "anonymous address" can be uses. In that case a special flag is used to avoid confusion between ICAO addresses and anonymous addresses.