|There are many types of UI implementation, using a single Form, that you could use here.
If you describe the purpose of this application more specifically, I think you can get more specific advice on which UI type might be be best.
1. do the UI buttons on the left side, always remain fixed, and always have the same title (display text) and function ?
2. when you select one of the possible right-side UI's required: does the amount of horizontal, or vertical space on the Form need to change in order to show all the required controls. That leads to:
3. can the single Form change in Size, or must it remain fixed in size ?
4. finally: can we assume that the varying contents on the right side are going to be a set of facilities, each one with its own unique controls and/or content ?
I will assume in this response you are working in Windows Forms.
For example, here's one idea:
1. You might consider using a SplitContainer control on your form, where the left buttons go on its left panel, and depending on the choice in the left panel, the right panel contents change. A SplitContainer gives you the option to expand the size of the right panel, if it requires more, or less, horizontal space.
The big questions here, to me, are: what are the "units of whatever" that appear on the right side, and do they ever effect what appears on the left side ?
Be happy to respond in more detail if you'll provide more detail here.
<color>"When it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images." Niels Bohr