What does an 'X' mean in the DOT hazardous materials segregation table under 49 CFR 177.848(d)-(e)?

49 CFR 177.848(e)

Confusing 'X' with a distance requirement like 'A' or 'O' could lead a driver or loader to believe that adding space or a barrier makes an 'X'-rated combination acceptable, when in fact no amount of separation legalizes it. Those two materials simply cannot go on the same vehicle at the same time.