Formula for a Drop-Down List

I have a spreadsheet that contains a comprehensive list of all of our vehicles and equipment. Within this spreadsheet exists all of the information that we have gathered about each item. Each item belongs to a specific "department". Each "department" has a "sub-department", and an "internal sub-department". An example would be the "Electrical" portion of the "Facilities Maintenance Department", or "Department: Facilities", "Sub-Department: Maintenance", "Internal Sub-Department: Electrical".

In an effort to organize this in a more efficient manner, I have assigned "codes" for each one. So, for example, Facilities is "C", Maintenance is "5", and Electrical is "a", so the code for the "Electrical" portion of the "Facilities Maintenance Department" would be C-5-a. All of this is done on a separate page of the workbook, labeled "Departments".

What I am attempting to do here is this: On the main page (the page that holds the rest of the Vehicle & Equipment information), I have a column in my table that is labeled "Dept. Code". I am trying to make it so that when an employee selects the drop-down box for Vehicle #6, it lists the options of departments/sub-departments/internal sub-departments to choose from. BUT, when a group is selected, ONLY the "Dept. Code" shows.

Can anyone assist me with this one???

I am sorry for wasting everyone's time with this. The more that I think about it, the more I realize that it is way more advanced than I was prepared for. So for now, we will just be doing it the old fashioned way.

Thanks anyways!

DG