What does this have to do with clipboard? Dimensions is about where there is content in the sheet, and it contains in many documents incorrect information which is why ExcelDataReader does not use it.

What is the intended use case for this?

When copying a range of data of an worksheet from Excel to the clipboard (e. g. with Ctrl+C), Excel saves an document to the clipboard containing only the data at the location of the selected data. For example, if you select the range D3:E4 and then press Ctrl+C, the clipboard will contain amongst others an Excel document in Biff12 format, where only the range D3:E4 is filled with values. To be able to find the copied data range, the mentioned properties are filled accordingly by Excel. In this example, FirstRow will be 3, LastRow will be 4, FirstColumn will be 4 and LastColumn will be 5. Therefore when loading this Excel document from the clipboard using ExcelDataReader, these properties contain the necessary information to determine the location of the copied data range.

I think this should be exposed as a Dimension property with a comment stating that it is unreliable. To make it clearer what it is.

And test cases.

I refactored the code to expose the clipboard-selection as property Dimension and added two tests (for Biff8 and Biff12).