At the meeting last Tuesday there was a question about the SSID used for APRS.

In an AX.25 network, each station must have a unique ID. Since a operator may have more than one station on the air at the same time, the protocol allows for a "Secondary Station Identifier (SSID). The SSID is a four-bit integer (0-15) that uniquely identifies
multiple stations using the same amateur call sign.

APRS originally repurposed existing TNCs, many of which had limited abilities, so there were some conventions adopted to use the SSID to identify the “type” of station – those conventions are still used (though not require)  today.

The most commonly seen SSIDs are:
0 (the equivalent of just your callsign with no SSID) for a primary station
9 for a mobile station
7 for an HT (historically the Kenwood TH-D7)

If you search the internet you’ll find a variety of lists for what each SSID represents; the “original” list (by WB4APR, the creator of APRS) can be found at  https://www.aprs.org/aprs11/SSIDs.txt:

-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