From Ubiquiti Wiki

AirOS Firmware Naming Convention

Legacy Products

 * PowerStation
 * LiteStation
 * NanoStation
 * MiniStation
 * WispStation
 * NanoStation Loco
 * PicoStation
 * Bullet
 * AP1000

[product][frequency].[chipset].v[version].[date].[buid][suffix].bin
   |         |           |          |        |     |      |
   |         |           |          |        |     |      |- Describes fw specific.
   |         |           |          |        |     |         Not used for hardware with 4MB flashes.
   |         |           |          |        |     |         -8M for 8MB flash hardware
   |         |           |          |        |     |         like PICO Station.
   |         |           |          |        |     |        
   |         |           |          |        |     |------- Build number.        
   |         |           |          |        | 
   |         |           |          |        | ------------ Build date.
   |         |           |          |
   |         |           |          |---------------------- Version string.
   |         |           |                                  Example: 3.4-rc
   |         |           |             
   |         |           |--------------------------------- Hardware chipset code based on.
   |         |                
   |         | -------------------------------------------- Device frequency code.             
   |                                                        2 - 2.4GHz device.
   |                                                        5 - 5GHz device.
   |         
   | --------------------------------- Product code.
                                       XS - Universal firmware for any device which has v3.0
                                            or newer firmware installed.
                                       LS - LiteStation firmware for device with v2.2.1 and older.
                                       PS - PowerStation firmware for device with v2.2.1 and older.
                                       MS - MiniStation firmware for device with v2.2.1 and older.
                                       WS - WispStation firmware for device with v2.2.1 and older.
                                       PICO - an alias for XS fw with 8MB flash devices.