When you have a full backup and a set of log file backups, a log chain
is maintained using the LSN (Log Sequence Number). If you want to do a backup
without breaking the log chain, then do a copy-only backup.
If you do not do a copy-only backup, the log chain is broken and the
backup you take will be the latest full backup. This means that the previous
log backups cannot be applied to the newly taken full backups. The log chain is
mostly maintained for point in time recoveries or log shipping scenarios.
For example: say you have a backup scenario that takes full
backups every 6 hours (midnight, 6 am, noon, 6 pm) and log backups every 15
minutes. A request comes in at 9 am to have a copy of your DB placed on a test
server. You want to take the backup without breaking your log chain or
disrupting your backup jobs. This is when a copy-only backup is taken. The copy
only backup will not disrupt your regular backup sets.
