"full" Creates one file for all tables (default).
"tables" Creates for each table within each database a seperate file.
${uri} [ _ ${comment}] [ _ ${datetime}] .sql [.bz2]
With --dump-type=tables:
${uri}.${table} [ _ ${comment}] [ _ ${datetime}] .sql [.bz2]
drush -r /path/to/drupal -l example.com multi-sql-dump --bzip2 --comment=before-update-to-6.15 --target=$HOME/db_backups
The result file will be bzip compressed and stored in 'db_backups' in your home directory.