The sub-categories are established by the tables in the CatTables worksheet.  You can modify subcategories used by the Inventory user forms by modifying these tables.
Adding a Sub-Category
Click on the screen capture to view at full resolutionOne of the easiest modifications to the Inventory is to add a new sub-category to an existing category.  In this case I want to inventory the cameras I have on board, but there is no suitable sub category.  So I've decided to add a "Camera" sub category to the "Entertainment" category.
The initial state of the CatTables worksheet (red arrow on the worksheet tab) is shown in the screen capture on the left.  The subcategory table for the Entertainment category is highlighted in the red rectangle.
Click on the screen capture to view at full resolutionTo add a new sub category I just type "Camera" in the cell below the last entry in the Entertainment table.
That's all there is to it, the Camera sub-category has been added.
Click on the screen capture to view at full resolutionI have opened the "Enter Form", clicked on the Entertainment category and displayed the drop down sub category list.  The new Camera subcategory is highlighted by the red rectangle.
However, the list is not sorted in any way.  If this were a large group of subcategories it might be difficult to find a specific sub category.
For that reason I normally sort these tables.
Click on the screen capture to view at full resolutionSorting of the sub categories is accomplished using the standard sort feature in Excel.  However this needs to be done carefully to insure the only cells you are changing are those associated with the Entertainment sub categories.
In this screen capture I have used the mouse to select all of the item in the Entertainment table (red rectangle) but not the title of this table.  I then used the Data/Sort menu item or the ascending sort button the Excel toolbar to initiate a sort.
Excel likes to sort all contiguous cells and has displayed a Msg Box asking if  want to expand the sort area in that manner.  Be sure you check the box the says, "Continue with the current selection" (red arrow) before you click OK
Click on the screen capture to view at full resolutionNow the items have been sorted in ascending alphabetical order.  The next time one of the user forms is opened and the Entertainment sub categories display they will also be in ascending alphabetical order.
Renaming a Sub Category
Renaming a sub category is as simple as typing over the entry in the CatTables worksheet with the new name.  However, if you have already assigned a number of inventory items to this sub category it is best to follow the instructions for renaming a locker name or ID, and re-assign those items to the the new name by adding that name to the sub-category list.  Then when all items have been re-categorized remove the old category, see below.
Removing a Sub Category
Removing a sub category can also be as simple as deleting the name from the list in the CatTables worksheet.  If inventory items have already been assigned to this subcategory then follow the precedures referenced above and described for removing a Locker from the configuration.