When combining demand sets, Junctions effectively works out the actual number of HVs for each demand set and then combines them all together into a single set. For example, say one demand set has 10% HVs and a flow of 1000 veh/hour and another set has 5% HVs out of 500 veh/hour. Junctions will know that the flow contains 125 (i.e. 100 + 25) heavy vehicles and 1375 light vehicles. This is equivalent to 250 PCUs due to the heavy vehicles (one HV is equivalent to two Passenger Car Units) and 1375 PCUs due to the light vehicles, giving a total of 1625 PCU.
In Junctions 8 and above, if you set up demand set relationships, you can see the calculated HV percentage in the resultant demand set by viewing the Vehicle Mix screen.