4. Protecting Sheets and Workbooks

Excel has several levels of protection, and each solves a different problem:

• Sheet protection (Review › Protect Sheet) stops cells from being changed. By default every cell is "locked," so first clear the Locked checkbox on the input cells (Format Cells › Protection) and only then turn protection on. Users can then change just the parameters, not the formulas;
• the same dialog has a Hidden checkbox: on a protected sheet, the formula bar will be empty for those cells;
• Allow Edit Ranges gives different people different areas to edit;
• Protect Workbook Structure (Review › Protect Workbook) prevents adding, deleting, renaming, and unhiding sheets;
• Encrypt with Password (File › Info › Protect Workbook › Encrypt with Password) is the only one of these that actually encrypts the file. A forgotten password can't be recovered;
• organizations often use sensitivity labels and OneDrive/SharePoint permissions instead of passwords.

Sheet and structure protection are safeguards against accidents, not against hacking, and they are easy to remove with outside tools. Data Validation (drop-down lists, number limits) complements protection by stopping a wrong value from being entered into an allowed cell.