Do you mean MDC or GE-STAR signalling? If you mean the digital signalling (MDC or GE-STAR) I think it's either the radio, or preference, I know with my MT1000's you can either have the signalling either at keyup or after the radio is dekey'ed.
If he's talking about the digital signalling like I asked before, then the ID tones are usually that screech or other sound you hear at the end or beginning of a transmission. The sound usually contains Unit ID information, etc, in a digital format that can be read by a dispatch console.
ANI (automatic number identification). It's used to send out the radio number so you know who is talking, who pushed the man down button or who has a mic keyed accidentally.
They can be programmed before, after or both before and after each transmission. Some don't like it at the begging of the transmission because you have to pause slightly before talking while it sends the id. Some don't like it at the end for other reasons.
In our area, different departments use different formats. MDC1200 MDC600 DTMF & Fleetsync. Some of the faster formats can be used to send preprogrammed messages to one radio such as in-service or call the office. Kind of like a poor mans MDT.