This one is hard to explain in plain English. Here's the scenario...
Door family with nested window families as sidelights.
Visibility parameter applied to the windows for left and right options.
When inserted into a wall (in a project file -.rvt) checking the sidelights on and off will not leave a hole where the window was. Rather, the wall will 'heal'.
The key is this:
In the window family
Category and Parameters, Shared (check box) is checked.
Thanks to Jeff Steinhauer from the AUGI forum for helping me on this one...