@viridian Just jargon used on forums. If we want to be technical it is a system that is targeting a standard. What standard that is that people on forums refer too, I have no idea
I have had my HRTF stuff modeled so I know my idea of neutral is a -0.75dB/Oct tilt from 20Hz to 20kHz. so, using room treatment, speakers that have the best baseline balance and using DSP to add finishing tweaks has helped me achieve this tilt in my 2.2 setup
Reference also refers to how effortless a system can get to 80dBSPL sustained over a passage of a song and how it can handle dynamic transients which means distortion and compression rejection should be a good thing for that system to have.
Outside of the technical delta which defined the tolerance amongst humans for what is perceived neutral, anything that is said to be reference is just what someone personally finds appealing.
That’s my 2 cents though