Clarifications for EML-310 Voter registration

In reviewing EML-310 for US UOCAVA purposes - need some help clarifying the following items:

ChannelID - there is no annotation on this - my sense is this is the device identifier for the channel used to provide the information about the voter.

Checkbox - this one I'm uncertain about - does anyone know what this does?

Source - there is no annotation on this - my sense is this is the authority name who is providing the information 

Qualification - again no annotation - but assuming this is where can put in the type of qualification(s) - e.g. UOCAVA, overseas, military, merchant marine, spouse, sibling, and so on.

Thanks, DW