You might want to use data from more than one spreadsheet in your AppSheet app. There are two ways to do this. You can add multiple standalone spreadsheet files as AppSheet tables. It is also possible to create AppSheet tables from different worksheets within the same workbook.
To be clear on terminology, a single dataset used by AppSheet is called a table. The source of a table is a single spreadsheet. A workbook is a file that contains one or more worksheets; a worksheet is a single spreadsheet that contains cells organized by rows and columns.
Either approach works just fine, depending on your circumstances. The performance of the two options may differ based on the specific content. The intuition is simple-- the AppSheet backend has to download the entire spreadsheet file but if the same file (workbook) is referenced multiple times (via multiple tables referencing individual worksheets in the workbook), the backend can be efficient and download it just once. On the other hand, multiple small files can be fetched and processed efficiently in parallel.
In most cases, you should pick whichever approach is convenient for you to manage. However, if you use only two worksheets out of a spreadsheet file with a hundred large worksheets in it, you'd be better off splitting those two off into their own single files.