Rob shared this with us in microsoft.public.excel.programming:

Hi

Is there a way that I can describe the paraeters of my function so
the user will the description when he clicks on the "black equals"
button to the left of the formula bar?

Not with default Excel, but Google for "FunCustomize" by Laurent
Longre. It's a combination of a dll and an add-in (xla), iirc.

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Amedee Van Gasse using XanaNews 1.17.4.1
If it has an "X" in the name, it must be Linux?