I'm not sure, but as they are associated with the dressage I think DEM means dressage error marks, and that would fit with the pattern of 2s and 6s, eg 1 or 2 errors of test.
And by similar reasoning, DEC may be dressage equal collectives- eg, if 2 people are on same score, same distance from optiumum time xc then they'll detrmine the result on collectives?
i think DEC = Dressage error Course and DEM= Dressage error marks as now instead of being eliminated for not wearing gloves, not dropping stick or leaving boots on you get penalties instead.