I don't know for certain if the spec at MacOSForge is definitive, but according to http://alac.macosforge.org/trac/browser/trunk/ReadMe.txt
3. From one to eight channels are supported. Channel orders for the supported formats are described as:
Num Chan Order
1 mono
2 stereo (Left, Right)
3 MPEG 3.0 B (Center, Left, Right)
4 MPEG 4.0 B (Center, Left, Right, Center Surround)
5 MPEG 5.0 D (Center, Left, Right, Left Surround, Right Surround)
6 MPEG 5.1 D (Center, Left, Right, Left Surround, Right Surround, Low Frequency Effects)
7 Apple AAC 6.1 (Center, Left, Right, Left Surround, Right Surround, Center Surround, Low Frequency Effects)
8 MPEG 7.1 B (Center, Left Center, Right Center, Left, Right, Left Surround, Right Surround, Low Frequency Effects)