Stepping limitations?

Having just finished book 3, I'm puzzled by what goes with a stepper when they step and what doesn't. Clothes and anything or anyone carried (so this is somehow constrained by footwear?) seems to be given. So if the stepper is in a building, say, the building doesn't go with. Twains step because they are sentient beings in and of themselves - but how come the gliders and the Prince's sand-yacht step? What are the parameters of 'what gets stepped'?