Aside from the object list and current guests, the sidebar shows you a list with vacant unit types:
The list depends on the time frame defined at the top right corner of the reservation schedule and is adjusted automatically when the time frame is changed.
You can turn this automatic display off in the Preferences. This can be a good idea if you don't need that information and are having problems with a slow-running program.
It is also refined depending on which objects you have selected and deselected. A deselected object's units will not be counted towards "available".