The TeXbook defines in chapter 24 (page 270 in my edition) the notion of
<muskip>, the first two being used in horizontal and vertical mode, while the other ones are used in math mode.
However this parallel between math mode and other modes is broken by registers: only
\muskipdef exist, and not
\skip 0 and
\muskip 0 exist, but not
The only primitive that manipulates mudimens seems to be
\mkern. Why are
<mudimens> so special?
EDIT: extra question. I'm asking this because I try to be able to grab the argument of any primitive. To grab a
<dimen>, for instance, one can use
\afterassignment\DoSomethingWithMyDimen \MyDimen=. Given that TeX does not have
<mudimen> parameters, I cannot use the same trick for that. I cannot use a muskip either, because it may mistakenly grab a stretch or shrink part which
\mkern would stop at. It may be that I can use a
\count register in some cases, but not all: how could I distinguish between cases like
\mkern 1mu and
\mkern \MyMuskip (with automatic coercion)?