I.e., the online backup will be as of the instant the backup is started and activity will be frozen while the BI is backed up.
An online backup allows you to keep users running, even though there may be some performance hit. An offline backup allows you to complete the backup process faster. E.g., many years ago I had a customer that wanted to do on-line backups so that they could start an overnight job queue and go home. But, as the business grew, the time to do the online backup and the time to process the overnight job queue, which was slowed by that backup, got so long that the overnight queue was not done by the time the east coast warehouse started work. They found that they could do an offline backup and then start the job queue and the whole thing got done much faster. Of course, we had fewer squirrels in the server back then.