Area-of-effect buffs - when to measure

[Squirrelmaster]

I'm trying to remember if there's a rule or FAQ about this:

If I use an ability which provides all units within a certain range with a boon, for example the Treelord Ancient's Command Ability (re-roll save roll of 1 for units within 10"), do I check which units are in range when the ability is used, or when I want to apply it?

So with the Treelord example, is it "all units that were within 10" of the Treelord in the hero phase (when he used his ability)", or "when a unit takes a save, if they are within 10" and the Treelord used his ability in the last hero phase"?

Is there a general rule for this, are there specific exceptions, do some abilities work work one way and some the other?

Another example is Neferata's command ability.

As I'm reading it, you measure when you want to use the effect, not when you activate it.

[BaldoBeardo]

You've got the right idea - generally it's when the effect of the ability is needed, although some abilities differ. So in your example, Treelord provides +1sv to units if they are within 10" when they make the save.
Just follow the wording and you shouldn't have problems.

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[RuneBrush]

Yup, @BaldoBeardo has sound advice, the key is in the ability wording.  

There are a few different varieties of buff.  For example, a piece of scenery that is "Inspiring", buffs a unit in the Hero phase to have +1 Bravery until the following hero phase - so that unit received the buff and carries it around with them.

The Khorne Skullgrinder grants all Khorne Mortal units within 6" +1 Bravery, it's possible that some models in a unit die in the combat phase resulting in the unit being 8" away and as such losing the buff.  This is the same as your Tree Lord example.

It's always worth noting too that you remove models from a unit's attack in "one go", so if one set of attacks inflicts 2 mortal wounds and 4 normal wounds, then don't remove anything until after you've rolled for saves (otherwise the 2 mortal wounds could result in the unit being farther away than 10")