Cecil wrote,

I'm not entirely certain, but I believe the H-parameter analysis,
the Y-parameter analysis, and the Z-parameter analysis all use
the reflection model and are valid for impedance discontinuities
in transmission lines.
--
73, Cecil http://www.qsl.net/w5dxp



Those are all examples of two port parameters, good for your basic linear,
time invariant system. (While you're at it, don't forget the
transmission parameters, Cecil.) They'll work with any old lumped
or distributed system as long as it's linear, has two ports, and is
time invariant.

You can't really use S-parameters to prove reflections exist because
S-parameter theory assumes reflections exist even when they don't, as in the
case of
transistor characterization - much like calculating the VSWR in a
zero length transmission line. They're good for analysis, though, as long
as you don't get carried away and look on them as proving something about
God, the Universe and Everything.
73,
Tom Donaly, KA6RUH