flybubba,
Do you have any SAT indication, or RAT alone?
Conventional wisdom holds that the iceing range is from SAT -40°C to 0°C.
The upper limit is usually specified as a TAT or RAT equivalent to 0°C at the highest Mach number where 0°C is likely to be encountered. The reasons for using RAT/TAT is that it is a fairly good indicator of skin temperature, and thus the aircraft's propensity to form ice if water droplets are encountered.
The lower limit is typically expressed as a SAT, as an 'atmospheric properties' value representing the lowest temperature at which super cooled water droplets can exist. As temperatures of -40°C can exist over virtually a 30000 foot range of altitudes, with great variation of Mach Numbers in that region, SAT is the only feasible way to express the temperature. The variation in RAT or TAT for the lower limit would be enormous.
Thus, if you don't have SAT indication as well as RAT or TAT, it is erring on the side of safety to specify "all temperatures below +6°C RAT".
But then again, perhaps you do have SAT indication, in which case I'd just have to say that Mr. Douglas plays the game more conservatively than does Mr. Boeing.
Regards,
Old Smokey