There are situation where we want the window to be divided in
several wings. Step 3 allows you to set the number of vertical
wings and/or the number of the horizontal wings that you want the
window to have.
Once the number of mullions is set each one of them can be moved to
the desired quotations. The mullions can be set by
percentage (default), setting that will reposition them in
case the window dimensions are modified, or they can be set by
quotation, in whichcase they will not reposition
themselves
See also
Add Component Wizard
Step 2 - Frame Information
Step 4 - Opening Information