prost,
Before VBA there was the Excel macro language. Custom worksheet functions could be written in the macro language on an XML sheet.
VBA is much simpler (more straightforward) than the XML language. On the other hand, the XML worksheet functions could be assigned to categories other than "user". In fact, you could create your own categories. For example, I had an XML sheet with dozens and dozens of formulas from the AISC steel design standard, these were assigned to the "AISC" function category. Now that is a feature that I miss!
Nowadays, I tend not to use custom worksheet functions due to the nature of what I'm doing. But I had a need for it over the weekend. It was not clear to me that the function had to be in a "Module" rather than in the VBA code associated with a worksheet or the workbook. (Editorial opinion: I couldn't find anything in the documentation in 2007 and there is no apparent difference in these VBA "pages" once you're in the VBA environment, so what's the difference?). But I finally got that figured out.
The XML language is still available as a mostly undocumented feature. I think you can even continue to create new such sheets. Maybe that's a back door to getting those truly custom function categories, like "AISC", "ACI", "ASME", etc.
There are a few XML functions that I continue to use without even thinking about them. I just checked and in one of my major ASME workbooks is an XML macro for interpolation within tables of data, I got this from PC Mag around 1993.
(Note, the 'XML' does not refer to the 'XML' variant of 'HTML'!)
