Q:

What is ... variadic argument syntactically?

What does a C/C++ compiler think ... is? To be clear, I don't think this is a duplicate question becuase other stdarg questions are about "what are variadic argument lists/how do they work?" That's not my question.
I have looked through MSVC's include files and found stdarg.h, vcruntime.h, etc., but haven't satisfied myself yet.
Does the compiler see ... as an operator? A linker symbol? A macro? It can't be an identifier, because that source character (.) isn't allowed in identifiers.
If I had to guess, I'd say it's something akin to using __attribute__ macros or inline or register compiler "hints" to inhibit warnings/errors upon invoking the function with multiple parameters.
From ISO9899:

6.5.2.2 Function calls
Constraints
6 The ellipsis notation in a function prototype declarator causes
argument type conversion to stop after the last declared parameter. The default argument
promotions are performed on trailing arguments.

I suppose not everything needs to be nailed down exactly, but I was curious if maybe there was more technical information out there.

A:

A punctuator.
ISO 9899:

6.4.6 PunctuatorsSemantics2  A punctuator is a  symbol that has independent syntactic and semantic significance. Depending on context, it may specify an operation to be performed (which in turn may yield a value or a function designator, produce a side effect, or some combination thereof) in which case it is known as an operator (other forms of operator also exist in somecontexts). An operand is an entity on which an operator acts.

