What's an add-in? Glad you asked. An Excel add-in is something you add to enhance Excel's functionality. Some add-ins provide new worksheet functions you can use in formulas; other add-ins provide new commands or utilities. If the add-in is designed properly, the new features blend in well with the original interface, so they appear to be part of the program.
Excel ships with several add-ins. Some of the more popular include the Analysis ToolPak, Conditional Sum Wizard, and Solver. You can also get Excel add-ins from third-party suppliers or as shareware; my Power Utility Pak is an example.
Any knowledgeable user can create add-ins (but VBA programming skills are required). An Excel add-in is basically a different form of an XLS workbook file. More specifically, an add-in is a normal XLS workbook with the following differences:
i The IsAddin property of the Workbook is True.
i The workbook window is hidden and can't be unhidden using the WindowOUnhide command.
i The workbook is not a member of the Workbooks collection.
You can convert any XLS file into an add-in. Because add-ins are always hidden, you can't display worksheets or chart sheets contained in an add-in. However, you can access an add-in's VBA Sub and Function procedures and display dialog boxes contained on UserForms.
Excel add-ins usually have an XLA file extension to distinguish them from XLS worksheet files. However, this is not a strict requirement. An add-in can have any extension that you want.
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