Symbols are set by the symbol table character, symbol character, and overlay character in the APRS packet for most APRS stations.
The only APRS stations that currently rely on default-symbol-by-SSID (see below) are those stations that have no capability to send APRS-formatted packets. The only ones currently like this are MIM's and old generic TNC's that are "APRS-capable". In this case they send a complete NMEA string from the GPS and limited symbol information is carried in a separate part of the packet. Sending the ultra-long NMEA strings are starting to be considered bad practice anyway: Much shorter APRS-format, Base91-format, or Mic-E format packets are the norm. Please note that by default UI-View and UI-View32 PC client software will reject Mic-E packets unless this option is turned off.
So... 99.8% of the people out there will be setting their symbol in their APRS client software or in their configuration software for their tracker, and the SSID (see below) has no influence whatsoever on what symbol is displayed.
 SSID's (Secondary Station Identifiers)
An SSID is the number following the callsign, which can range from -0 to -15 on the air (internet-only stations have different rules). A -0 (dash-zero) station shows up in mapping as the callsign without the -0. For instance W7ABC-0 will show up as simply W7ABC. A person may have 16 different APRS stations on the air at one time using only the SSID's to differentiate between them.
It is important to select a different SSID for each on-the-air station. A common "newbie" mistake is to use CALLSIGN-0 for the home station -and- for the mobile station. If they then drive a slow circle around their house, where the house station is beaconing at the same time, it makes a pretty flower pattern on everyone's map. The symbol keeps darting from the car's location to the house location leaving a track line on the map each time. The fix for this is to set a different SSID for each station so that software can track each one separately.
Originally the protocol was to use CALLSIGN-0 for the house, -1 for the first vehicle, -2 for the next, and so on. The more modern protocol (as defined by Bob Bruninga (WB4APR)) assigns a meaning to each SSID; however local custom can vary quite a bit on this.
It has also been noted that Bruninga's modern protocol does not address the numbering of multiple mobile stations. However when one considers that FCC Part 97 rules do not allow operation without a licensed radio amateur at the control point, then the argument that a licensed amateur would need mutiple SSIDs for more than one vehicle is a bit specious.
Here is the modern protocol:
- -0 Home station, or a home station running IGate.
- -1 Digipeater, home station running a relay Digipeater and/or WX Digipeater
- -2 Digipeater on 70 cm
- -3 Digipeater
- -4 HF to VHF gateway
- -5 IGate (not home station)
- -6 Operations via satellite
- -7 Handheld radios (Kenwood TH-D7, Yaesu VX-8R, ICOM D-Star, etc)
- -8 Boats, sailboats and ships (maybe 802.11 in the future)
- -9 Mobiles
- -10 APRS-IS only - APRS with no radio
- -11 APRStouch-tone users (and the occasional balloon)
- -12 Portable units such as laptops, camp sites, etc.
- -13 Not defined
- -14 Truckers
- -15 HF stations
 Default Symbol by SSID
As mentioned above, choosing the symbol based on SSID is ONLY done by the client software when there is no other symbol information in the packet to draw from. Right now this includes only MIM trackers and old generic packet TNC's that send NMEA-formatted strings. These are rare and getting more rare on the air. See the "SYMBOLS WITH STAND-ALONE GPS TRACKERS" section on this page: [APRS Display Symbols] for information on this (again rarely used) technique of specifying symbols.