Which units does a diff_drive_controller output?

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Hi,

I am using Kinetic on Ubuntu 16.04 with a physical robot.

I am writing the code that interfaces the diff_drive_controller to my physical robot's hardware. I understand that the data the diff_drive_controller takes in is geometry_msgs::Twist (and only uses angular Z, and linear X) which it then converts to velocity commands for each one of the two wheels (the joints specified in the yaml file). My question is: what is the output from diff_drive_controller for each wheel (ex: radians/second, or meters/second, rotations/second, or something else)?

I know how to turn my wheels using code, and now I need to figure out what data I am about to start receiving from the controller. This will allow me to make sure the wheels do exactly what the diff_drive_controller is requesting.

Thanks.