If it's specific, then citation please? I didn't think 'using a power' was so well-defined. Similar to others, I'm also at work, and therefore AFB.
Does it go into explicit detail about how the 'Use a Power' action is a different and distinct thing from sustaining a power? No.
It also doesn't describe how it's a different game concept than the Walk action, shifting, dealing damage, conditions, or what a d20 is, but that stuff is pretty common sense too.
Sustaining is spending a specific action to extend the duration of a power. Using a power is activating a power. Apples and oranges, and it's obvious as to why.
Durations of effects do not cease when someone leaves Line of Sight (which isn't necessary for the effect to be applied to begin with). Therefore, it does not affect powers with sustained durations, any more than it affects powers with (save ends), until end of encounter, or any other duration. Using a power, on the other hand, follows the rules for using a power, starting with page 1, and ending on page 300 and something.
Don't mind if I don't bother to quote that when 'they are obviously not the same rules concept' applies so much nicer.
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But, here's some ad absurdum anyways:
Action Type
The next line of a power description begins with what type of action you have to take when you use the power.
This is in the context of 'how to read a power.' That's the 'Standard Action' part that's before the range of the power. If sustaining a power were using a power, then by -this- bit of rules text, you'd have to spend this action to sustain the power. Which is silly given that you're already spending a different action.
Sustain
...
If a power has a “Sustain” entry, you can keep that power active by taking a specified type of action (minor, move, or standard) during your turn. The “Sustain” entry tells you if a power has an effect that occurs when you take the action to sustain it. See “Durations,” page 278, for more about sustaining a power.
Do note here, that 'Sustain' mentions nothing about it using the power, it only says 'keep that power active.'
And lastly, some obviousness:
Attack Powers and Utility Powers
...
You can use at-will powers as often as you choose. You can use encounter powers many times during a day of adventuring, but you have to rest a few minutes between each use, so you can use them each once per encounter. Daily powers are so dramatic and powerful that you can use each one only once a day.
If sustaining a power were considered using it, than by using the encounter power, you've used up the number of times you're allowed to use that encounter power. That means when it's time to sustain it, you cannot 'use' that power, which means you cannot sustain it.
Therefore, the only powers available you'd be able to sustain would be at-will powers.
How many at-will powers have sustain? How many encounter/daily powers have sustain?
Therefore, you cannot be using sustain powers, because otherwise, the rules
would not work.
