Frozen Synapse (Multiplayer Key)

My guess, and this is only a guess, is that it's the second key you get so you can give it to a friend. If you look at the part where you can buy the game, it says "Includes two copies of Frozen Synapse - Send the extra copy to a friend." There is only one place where it mentions the word multiplayer: '5 challenging multiplayer modes, including the innovative bidding-based “Secure” and “Hostage Rescue”' Nothing about that indicates you need a special key to access that mode.

Answer at the very bottom of this page:

https://www.steamgifts.com/go/comment/RP0xXJb