The party warlock has a wand of web.
He just leveled to 11th and is considering his choices for his 6th level arcanum.
Scatter (XGtE) allows him to teleport creatures to unoccupied spaces against their will, and he would like to be able to send them to a space with his web or an environmental hazard like fire, slime, or mold.
The PHB uses 'unoccupied' 49 times but never defines what an unoccupied space is, as far as I can tell, and neither does the DMG. The context of dimension door, and the figurine of wondrous power, make it clear that a space can be occupied by at least a creature or object, but do not explicitly say that these are the only things that can occupy spaces.
Does a spell effect or environmental hazard occupy a space?