|OK, now I think we have really come to the heart of the disagreement.|
We need to define Function vs. Formula.
If you Google Function vs. Formula, you'll find that many of us define a Function as those built in pieces of magic inside Excel such as VLOOKUP, SUM, ISNA, IF, etc. Of course, let's not forget about User Defined Functions (UDF) written in VBA.
On the other hand, a Formula is that thing that you type into the Formula Bar of a spreadsheet that (hopefully) Excel can evaluate. A Formula may or may not contain one or more functions.
e.g. All three of these are Formulas, but only 2 of them contain Functions:
So, to use your definition of "valid data" as something that does not produce an error when used in a Function, you would have to consider my data (A1) invalid since it will produce an error in the VLOOKUP Function, although it won't produce an error in my Formula.
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