What Donkey said is correct, but I want to explain a little more because this is subtle:
Yes, M4A is the container and it can contain both AAC and ALAC compressed formats. Which makes it rather confusing for a lot of people since you can normally use the file extension to figure out what's inside. At least in the audio world. In the video world all bets are off.
Anyway, MC has many many database fields (as you probably know) and one of the built in fields is called [Compression] . If you display this field for your M4A files, you will see that it shows whether the file is actually AAC or ALAC.
You can use this field to sort, search, build smart lists, build views, etc. However you want to locate or group your ALACs versus AACs, this is the field to use to do it.
Good luck.
Brian.