There's a lot of lingo around the wider web so this is one of a series of short definition posts.
Unlike page controls or overlay controls (the other two types of controls on the wider web) spatial controls are usually three dimensional and usually placed into the environment instead of sitting on the surface of the display.
For example, a spatial control could be a switch on a virtual lamp in a virtual environment seen through an immersive display or it could be a set of washing settings that hover around a washing machine when viewed through a portal display.
In the "Spatial Controls" section of the image below, the spatial controls are the sign that reads "PotassiumES" and the two hanging signs that read "Home" and "About":
Using Three.js to build a reusable border geometry for spatial UIs!
🌸 Updated potassium style system (KSS), now with margins!
🌸 The path to the vNext
🌸 A couple of new spatial controls
🌸 A brief intro to the existing samples
🌸 Building UI components that work in flat, portal, and immersive display modes