When I open an Excel function(e.g. IF function), message box that contains
the function arguments shows a description of the function as well as a
dscription of current variable selected. When I set the cursor into the next
variable, the description of this variable is shown.

How can I use this functionality in my own VBA functions?

FYI: I am particularly looking for a way to define description text in
relation to the variables. I have managed to change the overall description
text (i.e. of the function).

I am using Excel 2007