Yes discrete means each channel is independent of the other as opposed to a matrixed system like Dolby Stereo SVA optical where 4 channels are folded into 2 and extracted upon playback.
Originally 6-track mag was conceived as 5 front channels and 1 surround however the system evolved into 3 front channels, 2 surround channels (either split or mono) and the subwoofer (baby boom) channel. This carried over into the digital age with CDS, AC-3 and DTS using the same arrangement.
From reading the article Where Eagles Dare had a 4-track mix (L-C-R-S) prepared for 70mm and 35mm mag stripe prints (35mm mag is 4-track). Rather than make a dedicated 70mm 6-track mix the 35mm 4-track was carried over, leaving the LC/RC tracks muted. As this was before Dolby Stereo was introduced magnetic sound was the only way to present stereo sound in cinemas, the standard anamorphic 35mm optical prints were mono. Mag striping was a fairly involved and expensive process with a fair amount of wasted prints from incorrect application, so it was dropped as soon as an alternative came about.