What is Charon's ascending node in reference to? NASA's orbital elements given for Charon's orbit are given at https://ssd.jpl.nasa.gov/?sat_elem. The ascending node is defined as being "measured from the node of the reference plane on the ICRF equator". For Charon, the plane being used is given as the "Mean equatorial (IAU equator)". I'm assuming this is Pluto's equatorial plane. What then is Charon's ascending node in reference to? Normally this would be ecliptic vernal equinox, however this obviously cannot be it.

Some sources have the ascending node as undefined, but that then begs the question, what is the argument of perihelion in reference to if the ascending node is undefined?
I'm assuming this is Pluto's equatorial plane.


That is incorrect. Charon's right ascension of ascending node with respect to Pluto's equator is undefined. Seemingly paradoxically, it is well defined with respect to Earth's equatorial plane, and since that has become the universal plane of reference, that is what is used.



Charon and Pluto are tidally locked to one another. Their mean axes of rotation and orbital axes are identical. This means that Charon's orbit has zero mean inclination with respect to Pluto's mean equator, which in turn means that Charon's mean right ascension of ascending node with respect to Pluto's mean equator is undefined.

Charon's orbit about Pluto is however inclined with respect to the Earth's mean equator. In particular, it's inclined with respect to the Earth's mean equator at the J2000 epoch (noon Terrestrial Time on 1 Jan 2000). The Earth's mean equator at the J2000 epoch is the ICRF equator, to within a fraction of a milliarcsecond. The linked data clearly states that Charon's right ascension of ascending node is measured with respect to the ICRF equator.


Reference:

Brozović, Marina, et al. "The orbits and masses of satellites of Pluto." Icarus 246 (2015): 317-329.