Categories have keys automatically generated for them. They can be used by developers to identify a category regardless if its 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, with no objects selected:
Click on the Details icon next to the categories summary.
Select the Keys tab on the newly opened dialog.
You can now see the keys of your Categories, and even edit them.