People have used terms like
"free charge"
"mobile charge"
"static charge"
"induced charge"
"real charge"
et cetera.
In most cases, the macroscopic physics does not recognize the distinctions
that the words imply. (There are submicroscopic distinctions, but the
experiments being discussed don't probe that level of detail.) As a
concrete example: Given two capacitor plates with fixed geometry, I can
increase the capacitance
a) by filling some or all of the gap with an insulating dielectric
substance, or
b) by filling some (not all) of the gap with a conducting substance.
In response to an applied field, a slab of insulating dielectric will have
a charge on one surface and an equal-and-opposite charge on the other side.
If you reverse the applied field, the charges (macroscopically speaking)
will run to the other side verrrry fast. The behavior of a metal slab
wouldn't be much different.
The macroscopic capacitance (at reasonable frequencies) is remarkably
insensitive to the differences between
a) conduction in a metal,
b) displacement currents in an insulating dielectric, and/or
c) displacement currents in the vacuum.
To elaborate on point (c): Note that j (real current due to moving charges)
is not the only source term for the magnetic field. A capacitor can carry
an AC current even if there are no charges (moving or otherwise) in the gap.
The important difference between a conductor and an insulator has to do
with what happens to *injected* charges.
=========
Example #2:
People have spoken of "the charge on the capacitor" without making it clear
whether this is
a) A Kirchhoff-type charge distribution, treating the capacitor as a
two-terminal device (+Q on one plate, -Q on the other), or
b) A Kirchhoff-violating charge distribution, treating the whole
"capacitor" as a one-terminal device, i.e. a chunk of metal with an
unbalanced charge on it.
=====================
Suggestion: If you want to understand N-terminal capacitors (with N>2),
there is a perfectly good standard formalism: spell out all the charges,
spell out the mutual-capacitance matrix, et cetera. If that sounds
complicated, well, that's how the cookie crumbles. The real physics is
complicated. We can't make it simpler by using sloppy terminology.
Also: Yet again I wonder what is the pedagogic value of this dissectable
capacitor. AFAIK, there is no use for such a thing in industry or in a
real research laboratory. It looks like a way of amplifying all sorts of
junk effects. There *are* useful N-terminal capacitors, notably the
calculable capacitor at NIST, but they are as different from this Cenco
whatsit as they could possibly be.