Hopefully this helps, let me know if you need any more information.
There are three different spaces an app can be in. I'll run through each one. 1. Windows: A window is how most apps will work. For example, safari works in windows. It is a 2D plane floating in the air, with some 3D aspects, such as buttons, shadows, etc. Here is what the Developer website says about Windows: "You can create one or more windows in your visionOS app. They’re built with SwiftUI and contain traditional views and controls, and you can add depth to your experience by adding 3D content." 2. Volumes. A volume is another form of app type. This one is designed for 3D objects, but not for a 3D surrounding. For example, if there was a fish tank app, it would open in a Volume. It had depth, you can move it around, place it one the table in front of you, walk around it, etc. But you don't go in it. Another example would be opening a USDZ file. If someone texted you a 3D file, and you opened it, it would open in a Volume format. These apps are different than Windows in size, however, they can still share a space with other Window apps. So, you could be reading an article about the heart, while looking at a 3D model of the heart. Here is what the Developer website says about Volumes: "Add depth to your app with a 3D volume. Volumes are SwiftUI scenes that can showcase 3D content using RealityKit or Unity, creating experiences that are viewable from any angle in the Shared Space or an app’s Full Space." 3. Full Spaces. A full space is something that takes priority over all other apps. For example, if I wanted to use the SkyGuide app, and see the stars and planets around me, it would be a Full Space. No other apps can be opened when you are in full space. It takes up the "Full Space". Here is what the Developer website says about Full Spaces: "For a more immersive experience, an app can open a dedicated Full Space where only that app’s content will appear. Inside a Full Space, an app can use windows and volumes, create unbounded 3D content, open a portal to a different world, or even fully immerse someone in an environment."
