[ablob]

For meshed networks there is a secondary ID (with a name I do not know) that is used to distinguish between APs, since your device should only talk to at most one AP at a time. It wouldn't be surprising if they used that for finding the location, but marketing sells it as SSID matching as the people they want to sell it to are most likely not experts in networking.

[EvanAnderson]

The ESSID (Extended Service Set Identifier) is the human-readable thing you see. There is an underlying BSSID (Basic Service Set Identifier) that includes the unique identifier for the AP (its MAC address) your mobile unit is associated with.

On Windows you can see this (from an elevated context and, in newer versions, with location services enabled) by running: "netsh wlan show interfaces"

[kubeliv]

They could use the BSSID, which is unique per physical access point.