Used in churches as part of an anchorite's cell; a viewing space onto the outside world. Anchorites were voluntarily ( hopefully) walled up into a tiny room built against a wall of the church and this was their only access to outside; food and water could be passed through.Not sure how they managed toilet arrangements...
Also known as leper squints; the sick were kept away from the main body of the church and could only peep at the altar from outside.
Sign of division.. a barrier, a space that forms an impenetrable mass. Interesting how we design and manage space to include, welcome, withhold, repel, menace etc. Makes me think of homeless spikes and public benches specifically designed so that its impossible to lie down on them.