Position and velocity index

[sven1904]

What is the purpose of the different indices qIndex (position) and uIndex (velocity)?
And when should I use which, or when should I use none of them but the actual joint index.
I don't get why there are multiple indices and why they are different at all.

Some more details:
I'm working with pyBullet as well as Bullet's C-API.
And both seem to be working with different indices for their joint control methods.
When using pyBullet, it doesn't seem to make a difference if the joints are position, velocity, or torque controlled, all of the control modes use the 'standard' joint index.
Via C-API, it seems to depend on the control mode if qIndex or uIndex has to be used.
This is quite confusing for me. Can anyone enlighten me?