[whpalmer4]

They are all active backups, in the sense that you can go back to any of them and restore your database as it was at the time the backup was made. This is helpful if you don't realize there's a problem until several backups later! The date is put in the filename.

Backup policy can be set in the OmniFocus preferences, in the General section. You can back up once or twice a day, and optionally whenever you quit the program. OmniFocus will also make additional backups in "extraordinary" circumstances, such as when you replace or revert your database, switch sync methods, etc.

You can safely delete as many of the backups as you like, so long as you don't expect to restore from the deleted backups. They are independent files, each containing a full backup, so there are no issues with deleting one file making another useless. If you have Time Machine running, you can be more aggressive with your pruning of the files in the directory.