The function Excel is a wrapper around the Microsoft Excel SDK functions Excel4 and Excel12. It knows how to handle the memory protocols Excel uses so you don't need to.

Anything you can do in Excel and VBA you can do with this function. You don't need to know how to use this now, but when you are ready you can find a complete description of all the fuctionality available to you here.

The xll library function Register calls Excel with first argument xlfRegister to hook up your C/C++ functions and macros into Excel.