Quote Pivot Steel Exterior Door | EST-W4

A lite refers to each separately framed piece of glass on a window or door. A mullion is the line that forms a division in a window or door and this division can either be a real, or simulated.

True Divided Lites(TDL): the glass on the window isn't a single piece, but multiple pieces separated by mullions that go through the glass.

Simulated Divided Lites (SDL): the glass is a large, single piece with mullions placed on top of the glass, but they don’t cut through it.