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 are those stations that have no capability to send APRS-formatted packets. The only one's I know of that are currently like this are MIM's and old generic TNC's that are "aprs-cabable. In this case they send a complete NMEA string from the GPS and carry no symbol information.
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 as the plain-Jane callsign without the -0. For instance W7ABC-0 will show up as just 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 is beaconing at the same time, it makes a pretty flower pattern on everybody's maps. 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.
The most common setup is to use callsign-0 for the house, -1 for the first vehicle, -2 for the next, and so on. Some local customs vary this a bit.