You can use Validate to check your registered range for dimensions that the sidebar doesn't recognize (think "Rent" vs. "Rent Expense").

When would I use this?

I'd like to run a quick scenario analysis on Sample Co, so I've created the following table on Excel:  

After registering the range using Select Existing mode, I click Fetch Data.

While data has fetched to most of the range, I immediately notice that a few rows and columns are missing data entirely; is that because there's no data, or because the dimension name isn't recognized by Cube?

Using Validation

To check my range, I click Validate and a list of unrecognized dimensions is generated.

I immediately realize that Telephone and Internet isn't recognized because I changed its name to Telephone & Internet last month, so I update the cell by clicking the Edit icon. 

I also realize that Budgets should be Budget, so I edit the cell directly on the sheet and click Revalidate.

There's now only one unrecognized dimension; "Total". I don't expect Cube to fetch data to this dimension, so I click the  Ignore icon and toggle Show ignored dimensions off. The list is now empty!

Now that I've Validated my range, I click Fetch Data again and data is written to the entire range. 

Note: If the validate feature is not available (grayed out), please be sure the edit your range and ensure that both "Allow Fetching Data" and "Allow Publishing Data" are enabled.  Save your changes and the validate feature should be available for you.