It's neither positive nor negative. It just "heals" or "harms" outsiders with the selected alignment subtype. It doesn't go into much detail, but I've come up with a method to explain how it would function in-world. Normally, when channeling positive or negative energy, you're pulling the appropriate energy from the positive or negative energy plane and sending it into the selected creature types (living or undead). So, if you channel positive energy, you can "polarize" it to be absorbed by living entities to heal them, or to be absorbed by undead entities to harm them. But in the case of Alignment channel, you are no longer channeling positive or negative energy but, rather, channeling the four alignment energies (Good, Evil, Lawful, Chaotic). So, if you select Good, you are reaching into one of the Good aligned outer planes (instead of the positive/negative plane) and pulling Good energy to energize outsiders who run off of Good energy. On the other hand, to harm, you are reaching into the outsiders themselves and pulling the Good energy out of their bodies which, consequentially, causes harm. The same reasoning applies to Elemental Channel, except using the four Elemental energies instead of Alignment energies.
