A Custom User Sheet is simply that - a sheet designed to house your own unique, or custom, content. Generally this extends to the creation and use of tables and graphs that will eventually be merged over into an N1 report document. There are three Custom User Sheets included within every Narrative1 workbook to compliment your own intents and purposes.
You may rename each sheet by right-clicking on the worksheet tab and selecting Rename. Further, you can copy these worksheets as necessary by clicking Move or Copy.
If you are trying to merge tables you created from one Narrative1 Excel file into another - perhaps a newer version - it is required that those tables exist on one of these worksheets, and merged accordingly using Merge Workbooks.