I\'d better ask the question by example. Let\'s say I have UserControl and Window which uses this control.
I would like to design this control (named MyControl) in s
I think you want to set your UserControl's ControlTemplate with a ContentPresenter located inside (so you can define where the Content will be presented).
Your Custom UserControl:
Usage:

If you need multiple items in your content section, simply place them in a container like a grid or a stackpanel: