Plain glass is used for many purposes but there are three major uses. Window glass, picture glass and greenhouse glass. Each is different in specification and thickness and depends on its use.

Picture Glass is normally 2mm thick to avoid unwanted reflections in difficult lighting conditions.

Window Glass can be 3mm, 4mm or 6mm panes. This toughened glass should be used whenever there is a danger of people walking into or falling through a window, such as with single pane patio doors and French windows, and also with any large window that is mounted above the ground floor.

Greenhouse Glass will normally be 2mm for lightness, with the the exception of moving windows and doors which should be 3mm.

I am not sure if this was the answer that you were looking for, though I do hope that I was able to help.