Pretty easy once you've got the hang of it
You can use any of the brushes to clean windows.
Extreme brushes for the lightest possible weight.
Superlights for the maximum power.
Sill brushes for .... well sills I suppose or conservatory roofs and guttering.
Then each of the three ranges have variations to suit individual needs. You can of course use one brush for everything if you so desire.
I could buy the whole range but I bet I'd only use two or three max out of the lot.