1.9.5 Roll To Hit
The player must decide at this stage whether weapons
will fire with their AP or AT values if they have both. Then
roll a D6 for each shot being directed at the target
formation. You must roll equal to or higher than the
appropriate ‘to hit’ value to score a hit (eg, if the weapon
has an AT 4+ , you must roll a 4 or more to hit). The dice
roll is modified for the following reasons. However a roll
of 1 before modification is always counted as a miss.

The Barrage table lists the hit roll required to hit each unit under the Barrage template. Roll to hit all units (friend
or foe) under the template with the appropriate to hit values. In order to speed dice rolling we recommend rolling to
hit all units of exactly the same type together, and then removing any casualties from those closest to the enemy first.

And it does virtually the same thing as rolling individually, while speeding things up a bit in most cases

.......units of exactly the same type.......

Yeah, see what you can dig up E&C. there are still the two schools of thought in common practice on this one. Or that's my experience from playing a breadth of people from all round the country.

Under the description for units it says there are two broad types; Infantry and Armoured Vehicle.

Under the unit datasheet it says that "Type" is what target type the unit belongs to. After looking at a datasheet we see that this can be INF, AV, LV, WE, CHA, etc.

Writing "Units of the exact same type" suggest to me that the writer was thinking of more than two types otherwise he would've simply said "Units of the same type"......this seems to say that you should roll for each different target type.

However, remembering that the goal is to speed the dice rolling element, it wouldn't make sense to roll seperate sets of dice potentially for 4 or 5 different types of units. In this regard, I think it would simply be better to roll all AP and all AT seperately, as per the Barrage table column.

I guess it could go either way really, perhaps it is something to add to the 5 minute warm-up.

I've certainly had both interpretations at EpicUk tournements, I tend to go with roll all AT and Inf and not sperately for commenders et al as it helps stop character sniping with barrages that feels wrong to me, however I will use eiher if my opponant feels strngly, certainly one that should be FAQ'd