Q:

Which part of WifiInfo can be used to distinguish every unique access point in a big WLAN? 

Which part of WifiInfo can be used to distinguish every unique access point in a big WLAN? 
BSSID or MAC address? 

A:

BSSID of access point is essential to every device. 
You should get a WifiManager instance and then call 
wifiMgr.getConnectionInfo().getBSSID()

to retrieve the BSSID as a string, it is just the MAC address of your AP, format like 0C:DE:AD:1F:00:32

