Status Message Encoder. Each radio has a set of buttons that, when pushed, produces an (I assume) data code that indicates various messages related to the status of the unit.
Fire Apparatus mobiles have, (and I may have these reversed a bit, ie: 1 & 2, 3 & 4 may be backwards):
1 - In Quarters Dependently Staffed (two units share personnel)
2 - In Quarters Independently Staffed
3 - Responding with Officer
4 - Responding without Officer
5 - On Scene
6 - Available
7 - Unavailable
8 - Out of Service
They also have a set of buttons to indicate number of personnel on the apparatus, 1-8
The EMS units are basically the same, but 3 = Responding, 4 = On Scene, and 5 = Enroute to Hospital. Their second row of buttons are used to indicate which hospital they are transporting to. 1 = North Arundel, 2 = AAMC, etc...
Some portable radios also have SME capability and most have buttons to indicate when an EMS crew has arrived at a patient's side (initiated care) and when care is transferred to hospital staff which are not necessarily the same as time units Arrive On Scene or Arrive at the Hospital.
Hope this answers your question.