There should not be any restriction on the choice of boundary conditions @damiduz!
Can you be a little bit more specific about your question? Or do you mean the physical reason behind it? Afaik, the slip boundary condition uses a fixedValue with a zero velocity in normal direction and a zeroGradient in tangential direction - this BC does not make much sense for the floor in my opinion . For an explanation regarding moving walls, feel free to have a look at the post made by our fellow PowerUser @pfernandez: Moving Wall BC.
Hope that was what you were looking for. If not please let me know so that I can clear things up.
Best,
Jousef