I copy paste it here from the Salamanders thread:
Salamanders BTS needed the Warp Spiders, Revenants and Aspect Warhost to be whiped out.
But this led to a question: The Warp Spiders killed with First-Strike all Salamanders units within range of the Warp Spiders. We whereunsure if the supporting Aspect Warhost could then lend Supporting Fire? The Aspect Warhost was in range of the Salamanders but none of the Warp Spiders where in range to the Salamanders. We said yes which resulted to a +10 for the Eldar instead of a +6 for the Assault Resolution roll (with +0 for the Salamanders).

Another question came up in the first assault of the same game:
After the charge move (in this case an Air Assault) a Vulcan Dreadnought was within 15cm of enemy units. After allocating non-MW hits and making armour saves it was now time to roll for hits for MW-attacks. But now all enemy units within 15cm of the Vulcan Dreadnought were destroyed from non-MW hits. So could the Vulcan Dreadnought still use his FF-MW attack from it's Multi-melta or not?

If nothing is left in range after First Strike attacks have been made (and saved against), then no attacks would have taken place.

So the Aspect Warhost (not the Warp Spiders who actually assaulted) which where within 15cm of the Salamanders' survivors couldn't lend supporting fire?

So the Aspect Warhost (not the Warp Spiders who actually assaulted) which where within 15cm of the Salamanders' survivors couldn't lend supporting fire?

After the charge move (in this case an Air Assault) a Vulcan Dreadnought was within 15cm of enemy units. After allocating non-MW hits and making armour saves it was now time to roll for hits for MW-attacks. But now all enemy units within 15cm of the Vulcan Dreadnought were destroyed from non-MW hits. So could the Vulcan Dreadnought still use his FF-MW attack from it's Multi-melta or not?

To clarify a bit...

Support fire can only be allocated to units in range of the assaulting formation.

It doesn't matter if you have 100 units from support formations and the defender formation within 15cm range of each other.  Shots are only fired at the defender units which are directly involved in the assault.

1.12.5 Resolve attacks
In this case ‘directly involved’ means belonging to the attacking or defending formation(s) and in a position to attack.

Seems strange that if two large formations (say an IG Infantry Company and a Big Ork Mob) are in straight lines opposing each other 10cm apart, and the Ork Mob is clipped on an end by a Sentinel Squad, only 2 or 3 Infantry get to support.

Suppose it goes part and parcel with the whole "Orks can't fire on the IG either", but is still a little weird.

I believe this was one of the amendments added to stop 'gameyness'. Remember that formations providing supporting fire cannot be hurt, so in the past 'clipping' attacks were even more formidable because they could be set up to bring in huge amounts of supporting fire with limited risk. So supporting fire was restricted to only hit enemy units that were directly involved in the assault. An example might demonstrate this better

A1 A1 A1 A1 A1 A1 A1 A1 A1   A2  A2  A2  A2  A2  A2  A2    B B B B
C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1   C2 C2 C2 C2      B B B B

If formation "A" assaults "C", all "A" and "C" units can engage. Formation "B" is in position to support "A" by firing on the "C2" units (the "C1" units are more than 15cm away). The defender ("C") can allocate any hits suffered, so assuming the spacings shown, he is permitted to put them on the "C2" units first. If all "C2" units die, then "B" cannot provide supporting fire because there are no longer any units in range. However any "C2" units that survive will get supporting hits allocated to them and so are likely to die anyway. In any event, "B" started in position to support.

Instead of the above, if formation "B" assaults "C" with "A" in support, only the "A2" units can provide supporting fire as they are the only units in range of the "C2" units "directly involved" in the assault.