REGISTER.ID is an XLM function that facilitates connecting XLLs to menus, tools, event handlers, and other items that require you to provide the address of a subroutine. The REGISTER.ID function is a direct replacement for any function that requires a macro reference. An example would be a menu that needs to call the function MyFunc in an XLL named MYXLL.XLL.
In the second column of the menu definition table you would place REGISTER.ID("MYXLL.XLL","MyFunc"). When the menu executes the REGISTER.ID, if the XLL has already been registered, it calls the function. If the XLL has not been registered, Microsoft Excel registers it by calling xlAutoRegister and then calls the function.