Sorry, I am really a beginner, I do not understand "UDF" although I
suspect that the "F" stands for function? What "UD"is, I don't know.

Now, what's the point?
I would not make a function for =SUM(F10:F19)!
I gave you this example because it used two Worksheet Functions, but
other cells formulas  are so long  they take almost 3 lines.
It's OK as long as I am testing, but once I'm finished I'd like to
replace them by functions...
And, most of all, I would like to know how to, because I am trying to
learn!
Repeat:
The Visual basic  Help has a topic: List of Worksheet Functions
Available to Visual Basic.
What does that mean? how is it "available"?
I have a long sub that needs to make calculations on data that come
from the spreadsheet, and using some Worksheet functions  inside a
function, if it is possible, would help  me a lot.

If you tell me it is NOT possible, I'll stop trying, and I'll live
with it.

Or if you tell me I would have to re-write the whole code of the
Worksheet Functions, this would also have no sense!

What is the problem with "adding a layer that needs to be navigated
by the system"?