SSID RECOMMENDATIONS: It is very convenient to other mobile operators or others looking at callsigns flashing by, to be able to recognize some common applications at a glance. Here are the recommendations for the 16 possible SSID's (the limit of 16 comes from the 4 bits available in the AX.25 protocol. Note, The SSID of zero is dropped by most display applications. So a callsign with no SSID has an SSID of 0.
-0 Your primary station usually fixed and message capable
-1 generic additional station, digi, mobile, wx, etc
-2 generic additional station, digi, mobile, wx, etc
-3 generic additional station, digi, mobile, wx, etc
-4 generic additional station, digi, mobile, wx, etc
-5 Other networks (Dstar, Iphones, Androids, Blackberry's etc)
-6 Special activity, Satellite ops, camping or 6 meters, etc
-7 walkie talkies, HT's or other human portable
-8 boats, sailboats, RV's or second main mobile
-9 Primary Mobile (usually message capable)
-10 internet, Igates, echolink, winlink, AVRS, APRN, etc
-11 balloons, aircraft, spacecraft, etc
-12 APRStt, DTMF, RFID, devices, one-way trackers*, etc
-13 Weather stations
-14 Truckers or generally full time drivers
-15 generic additional station, digi, mobile, wx, etc
* One-way trackers should best use the -12 one-way SSID indicator because the -9's usually mean a ham in full APRS communication both message and voice. The -9's can be contacted by APRS message or by Voice on his frequency included in his beacon, or on Voice Alert if he is in simplex range. The -12's are just moving Icons on the map and since they have no 2 way communication for ham radio they are not generally of routine interest to other operators.
OBJECTS or INTERNET: In addition, Objects or internet generated stations can have any SSID, not just the original 16, since Objects are not constrained by the AX.25 header and can have a 9 byte name. Here are some common OBJECT/Internet SSID's:
63 for PSK63 HF stations
tt for APRS TouchTone users (DTMF)
ID for RFID
A through -Z for Dstar
Source: APRS SSID Guide