Here's the refined rules for selecting targets and splitting your fire between enemy units.
Shooting: Target Selection
Selecting a target for you’re unit’s shooting isn’t as simple as you might think. In the confusion of battle, it’s easy for a group of soldiers to miss something important and focus instead on more obvious threats. It’s up to your unit leaders, therefore, to keep their wits and remember what’s really important to the mission.
Any enemy unit within range and Line Of Sight of the firing unit is eligible as a target for your unit’s shooting.
Friendly units may not be targeted.
Without any direction, the soldiers in a unit will normally confine their shooting to the most obvious enemy target. It’s up to the Unit Leader to understand the tactical situation and ensure that his troops don’t fixate on a target that’s not really all that important to the success of their mission.
A unit with a leader may target any one enemy unit within Line of Sight to the unit’s leader model that is also within range of at least one of his unit’s weapons.
A unit without a leader may only select an enemy unit as a target so long as a majority of the models in the firing unit can draw Line Of Sight to that enemy unit. If there is more than one enemy unit to which a majority of your unit’s models can draw Line Of Sight, then they must select the closest of those units as their target.
Splitting A Unit’s Fire
A unit with a Leader may split it’s fire amongst two or more enemy units.
First, declare all of your unit’s targets and which of your models will fire at each of those targets.
Then, for each target after the first, roll a Ld check. If the check is passed, the models that you declared would fire at that target will be able to do so. If the test is failed, those models may not fire at their selected target. They either failed to understand their leader’s instructions, or they did not acquire the target in time to fire their weapons at it.
Once you have determined which targets may be fired upon, you may then fire on those targets in any order that you wish.
Units without a leader may not split their fire. Nor may any unit with the ‘Swarm’ rule, as their targets are selected through the instincts of the group as a whole rather than the orders of an individual leader.
Marking Your Targets
When you target a unit for shooting, you will need to mark one of it’s models as the primary target model within that unit. This will be important later when you are determining what you will need to roll to hit the unit. It will also be needed when your opponent has to allocate the wounds to his unit from your shooting.
The marked model must be within your unit leader’s Line of Sight. Or, if your unit has no leader, within the LOS of a majority of the firing unit’s models.
The easiest way to mark a model as the primary target is to use a die placed next to the model. Preferably one that’s a different color or style so that you don’t accidentally pick it up when you’re rolling your shooting attacks.