rc_control controllers modes

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Dear all,

I am currently checking the ros_controllers and I am not sure I understand the way the different modes are organized. From the ros_control wiki I read the controllers can be either:

1. effort_controllers
   1. joint_effort_controller
   2. joint_position_controller
   3. joint_velocity_controller
2. position_controllers
   1. joint_position_controller
3. velocity_controllers
   1. joint_velocity_controllers

From this I understand there are effort, position and velocity controllers which respectively take a desired effort, position or velocity as input and do their best to get the system state to this desired input (these controllers correspond to entries 1., 2. and 3.).

Now, what I do not understand is the meaning of sub-categories 1.1., 1.2., 1.3., 2.1 ... If I choose 1.2. for example, what's this controller? It takes a desired effort as input and probably does something related to a position as its name implies... But what?

Also controller 1.2. == 2.1. and controller 1.3. == 3.1., how is that possible? I guess it is related to my first question...

Anyone with a better understanding than me?

Thanks guys,

Antoine Rennuit.