MDC1200 is a 'digital signaling' technology widely used in analog, conventional two-way radio systems developed by Motorola. It is comprised of a 1200-baud, 32-bit data transmission.
The most common use is for PTT-ID (sometimes called ANI), where a unique 4-digit number (in hex) is transmitted either at the start of a transmission ("pre-ID") or at the end of a transmission ("post-ID"). Many dispatches consoles, as well as higher-tier Motorola radios, are able to decode and display the PTT-ID.
Some other uses of MDC1200 include:
Repeater access, via Data Operated Squelch
Emergency status notification: a button on the radio is pressed, indicating to the dispatcher that an emergency condition exists
Selective calling: a given radio on a network can be 'called' with a series of MDC messages
Status messaging: several pre-programmed messages can be used