Essential and Useful C API XLM Functions
The functions described in this section are Microsoft Excel callback functions that are particularly useful to DLL and XLL developers. Of these, the xlfRegister function is essential for XLLs and DLLs that want to register their functions and commands so that they can be called directly from Excel. The functions xlfUnregister and xlfSetName are used in combination to unregister DLL and XLL functions and commands.
Many more functions are exposed by Excel via the C API that are useful when you are developing XLLs. They correspond to the Excel worksheet functions and functions and commands that are available from XLM macro sheets.