The Stage tab lets you define objects that divides the whole Schedule into different physical Location, and are userdefinable top-level objects in the object-tree, each with its own bounding box and own origin/zero-point. Use multiple stages if your system covers multiple venues, rooms or stages.
A 3D Window (Formerly known as Stage Window) will visualize one Stage (or Room, Venue etc)
With the Stages concept you can visualize different venues/stages/rooms in different 3D Windows, within the same showfile
Try to get familiar with the Filter button. (shaped as a funnel in the top right) When your object tree grows, using Filters to only see what you want is a big help for navigation.
If you want to filter on your own terms, not only by Fixturetype, or DMX address etc, head over to the Layer and Class tabs, and create your person grouping preferences.
e.g. Layers: Floor, Roof etc
e.g. Class: Wash, Spot
assign the appropriate Layers and classes to your fixtures, and use Filters to easily find your Wash fixtures that are on the Floor.