The mullions of a door/window assembly are the edges between the grid cells. The primary grid and each nested grid have mullions that are defined separately from the other grids.
Specifying mullions for Door/Window Assembly nested grids
Mullion Assignments
A mullion assignment defines the definition used by each mullion. You can use one assignment to define all mullions, or you can create multiple assignments and assign different definitions to different mullions in any grid.
Do not assign multiple definitions to the same mullion because only the last definition assigned to the mullion is used.
Default Mullion Assignment
There is a default mullion assignment used by all unassigned mullions. You can modify the default assignment to be any mullion definition, but you cannot delete the default mullion assignment.