Categories have keys automatically generated for them. They can be used by developers to identify a category regardless of whether it's color or name has changed on the floor plan itself. For instance, when assigning pricing to categories.
To find out what your categories keys are, first open the floor plan with the Designer.
Then, you can either:
hover on a category for a second, which will display a tooltip with the key for that category.
click the information button next to Categories. Notice this button is only accessible when no object is selected.
That's all you need to know. Cheers!