From this page you can assign categories to the company level. Categories with the box to the left of the checked name are assigned. If the box is empty, the product is not assigned. To change the assigned status, click on the box and click the "Update" button.
For group or user assignment, select the appropriate tab and then choose "Categories" within that tab. Categories can be assigned at a higher level and there will be bold text to the right letting you know where they are assigned. You will see which categories are assigned at a higher level like this:
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Create a Category