As Netflix/Pandora/Crackle would be dedicated buttons whose only purpose is to launch their own channel -
query/apps This ‘query/apps’ returns a map of all the channels installed on the Roku box paired with their app id.
Then
launch takes an app id as an argument and a list of url parameters that are sent to the app id as an roAssociativeArray passed the the RunUserInterface() or Main() entry point.
So, you could launch Crackle, Pandora, or Netflix that way without needing the button itself.
I don't know what the A and B buttons are called as they aren't listed in the documentation.