Compatibility mode is a feature in Microsoft Excel that allows users to open and view workbooks created in older versions of Excel. When a user opens a workbook in compatibility mode, Excel displays it as it would have been seen in the version of Excel in which it was created. This means that any features that were added to Excel after the workbook was created will be unavailable. Compatibility mode is used automatically to open Word, Excel, and PowerPoint documents that were created in earlier versions of these applications.

Here are some key points about compatibility mode in Excel:

• Compatibility mode ensures that no new or improved features in Excel are available while users work with a document so that people who use earlier versions of Excel will have full editing capabilities.
• Compatibility mode also preserves the layout of the document.
• Compatibility mode applies to documents saved in or saved for older versions of Excel.
• Compatibility mode is useful when sharing workbooks with people who haven’t upgraded yet.
• Compatibility mode is enabled when converting a file from a different format, such as CSV or XLS, to the native XLSX format.

To leave compatibility mode, users can save the document in a newer Excel format or turn off compatibility mode. To turn off compatibility mode, users can click the "File" tab, select "Options" to the bottom left, select "Save" on the left side, and then change the "Save files in this format" drop-down menu.