This needs to be answered in reverse.
Platform height is how far you are standing off the floor, that is, distance between the ground and the upper platform board.
Tower height is the physical height of the tower itself, that is, distance between the ground and the top rail.
Working height is how high you can reach when you are standing on the platform. It is a common practice that working height is platform height plus 2 metres.
Please see below for a handy diagram.

