The Photon 2 is the development board in a featherwing form factor for the P2, a dual-band 2.4Ghz/5Ghz Wi-Fi and BLE device. It has a 200 MHz ARM Cortex M33 and has 4.5MB RAM with 8MB Flash.
The Photon 2 is the replacement for both the Photon and Argon. More information on Photon 2 can be found in our docs.