Reset Value In Dependent Cell When Another Cell Is Updated

1. I created a "Top Category Data Dependent Validation List for sub-categories” in the attached spreadsheet. The selection from the Category drop-down list in column B, then drives or limits the choices in the Sub-category drop-down list in column C.

Credits: To achieve the above technique, I used the "Dependent List Validation" document as a technical reference.
the document is available from http://www.ozgrid.com/Excel/dependent-lists.htm

2. If the user updates the Category selection in Column B, then the value in the corresponding row in Column C should be "reset to a ( NULL/Clear) value," as to automatically prevent any human-error in forgetting to also update the Sub-category data in Column C.

(Optional Reading--the following steps are not related to steps 1 & 2 above, just notes regarding other data validation process created in this spreadsheet

3. I also created another column (D) that gets automatically populated with the Category ID,
based on the sub-category drop-down list selection in C.

4. Also in this spreadsheet, a macro checks for any "missing required data
before allowing the end-user to save this worksheet. As an example, In Column A, if there is a "RecordId" that exists in the same row, then the macro checks if the user has entered required values in Column B, C, "AND/OR" D. If the end-user did not enter any of the required values,
then an error message pops-up with this message:

"Cannot Save this file due to Missing Required Data. Please review highlighted record and complete missing data."

Credits:

Special thanks to:
-OzGrid Business Applications for writing code for a UDF (function that returns last word) that I used/modified a little bit to achieve Step 3.

-Carl (member name" carlmack") for his help in the methods I used in Step 4 above.