[disc] represents the discnumber, or '[SETLEN]2,48,,[disc][]' is the discnumber with leading zeros (padded to two characters), so something like:
Code:
[disc]\[track] - [title]
or
[SETLEN]2,48,,[disc][]\[track] - [title]
I've never used Batch Ripper before, but I'd guess [discunique] simply represents a unique number, and is useful within the batch context for CD rips that lack online metadata, otherwise they'd probably overwrite any previous rip that also lacked metadata.