Bathroom: Installation of floor tiles on the walls

That can also depend on the tiles. There are some that eat drill bits. That can be costly for the craftsman.

The question of commissioning has not been answered by the OP. However, I gladly admit that I have already answered the question for myself ;-)

If it is a general contractor/GÜ, his subcontractors know his standard; they have adjusted their prices accordingly. If something changes in the usually owed tiles due to sampling, they will consider any additional effort in fastening (drills eating up) and charge for it.

Rhenish greetings from the road
Construction expert