Here's how to validate data based on the selection of a dropdown item from another cell in Excel:

Method 1: Using Data Validation with Indirect Function

1. Create your dropdown list: Define the dropdown list items in a separate location (hidden sheet or designated area).
2. Set up data validation for target cell:
- Select the cell where you want to validate the entered data.
- Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.
3. Set Allow and Source:
- In the "Settings" tab, under "Allow," choose "Custom formula" from the dropdown menu.
- In the "Formula" box, enter the following formula (replace A1 with the cell reference containing your dropdown selection and B2 with the cell you want to validate):

```excel
=INDIRECT(A1)<>"" AND (B2="") OR (B2>=LOWER(INDIRECT(A1)) AND B2<=UPPER(INDIRECT(A1)))
```

4. Explanation of the formula:
- `INDIRECT(A1)`: This retrieves the value selected from the dropdown in cell A1.
- `<>""`: Checks if the dropdown selection is not empty.
- `(B2="")`: Checks if the target cell (B2) is empty. This allows leaving the cell blank if nothing is selected in the dropdown.
- `OR`: This part handles data validation based on the dropdown selection.
- `(B2>=LOWER(INDIRECT(A1)) AND B2<=UPPER(INDIRECT(A1)))`: This ensures the entered data in B2 falls within the case-insensitive range defined by the dropdown selection (converted to lowercase and uppercase).
5. Error Alert (Optional):
- In the "Error Alert" tab, you can customize the message displayed if the validation fails (e.g., "Data must be within the range allowed by the dropdown selection").
6. Click OK: Save the data validation settings.

Method 2: Using VBA (For advanced users)

This method involves writing a macro using VBA (Visual Basic for Applications) to perform more complex validation rules based on the dropdown selection.

Both methods achieve data validation based on dropdown selection, but the first method using formulas is generally easier to implement for most users.