Surround processing can be broken down broadly into two methods: extracting and generating.

"Matrixing" typically refers to extraction. For example: sounds in a stereo recording that would normally phantom image at the centre of the soundstage are extracted and sent to a speaker at the centre of the soundstage. This is common to every matrix processing (PLII, Neo:6, Neural, Circle Surround, Logic7, etc). None of those surround modes ever adds anything to the recording, just matrixes out certain sounds to be steered to additional speakers. Think of matrixing as scaling: the number of channels in the source material need to be scaled to the number of speakers in your set-up.

This is different from room simulation, where you add spatial cues that have been generated to give the impression of being in a larger space. DSP processing from Yamaha and Sony, as well as DSX, fall into this category, since their goal is to give the impression of a different listening room than the one you're really in. This can't be done by matrix extraction, because steering ambient cues to different speakers doesn't change their arrival time (room size), just makes the ambience come from the appropriate direction (around you, instead of in front of you).