Richer apps can use more than one table or spreadsheet data set.
In the Editor>Data>Tables tab, click on '+Table' to choose another spreadsheet. If you need to use a worksheet from the same workbook you are already using in your app, simply define the sheet name (case included) in the Worksheet Name/Qualifier field. The sheet automatically gets added to your app when you save your changes. You can now define control views over this data as well.
Note that you'll need to create a new UX view pointing to the new spreadsheet to get that sheet's data to display in the app.
See a good example of multiple tables and cross-table references in the Order Capture sample.