Mercenary Tokens are an alternate form of currency added in the first part of the Sandstorm updates (Patch 1.3). Players earn Mercenary Tokens by attempting PvE and PvP missions of appropriate difficulty. Players are limited to earning a set number of tokens from a single source per day as well as a cap on the number earnable per day - a player may complete a number of High Security missions and find they cannot earn more tokens from PvE, but can still earn more from PvP missions.
Tokens are earned and capped at an Account level - switching (or creating) characters does not allow a player to earn more tokens, though it does allow a player to earn tokens on his or her Medic and spend them on their Robotics.
Mercenary Tokens are used to purchase epic varients of the core Devices. Each device is purchaseable for 200 tokens at their respective vendors in Dome City.
See Also - Conquest Tokens, another alternate currency added in Patch 1.3.