The Viewer tab provides one or more 3D windows into the scene described by the scene graph. Only locations that are exposed within the Scene Graph tab are represented in the Viewer - the exception being pinned locations.
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You can interactively modify parameters, on some nodes contributing to a scene graph location, using Manipulators within the Viewer. The manipulators available vary depending on the scene graph location selected, and the nodes that created it.
Katana does not assume a given scene scale. There are no physics solvers, like those in Maya, that need to know what real world measurement a unit represents. If you're using a shader library that has real world units for shade parameters, such as emissive lights with power per unit area, that's handled by the shader implementation rather than Katana.
The unit of measure used is up to you, because Katana equates any value as being equal to one unit in 3D space. The image shows a simple scene containing an object and a camera. If the large white square in the Viewer represents one unit of measure, so the smaller squares represent one tenth of that unit. So, if you measured on set in meters, one small square could be equal to a meter, centimeter, or millimeter.