Question: Quaternions: Why do I get two times the same generalized coordinate and some more questions

Doing multibody analysis on a ridig body results in

Turning quaternions on

Since I do not understand quaternions very well I was wondering why q0 is listed two times (why does the system uses 8 generalized coordinates for 7 degrees of freedom).

Ridig body help says about quaternions:Indicates whether the 3-D rotations will be represented as a four parameter quaternion or as three Euler angles.  Regardless of the setting, the initial conditions are specified with Euler angles.

Is that representation only internal? Can quaternions be probed?

To familarize myself with quaternions in MapleSim I would be greatfull for any examples using them. Are there any?

In which situations shall I use quaternions in MapelSim?

And what by way the stands q0slack in the above screen shot for?