Not 100% sure, but normally example 40@22 would mean, 40 snapshots are kept, and snapshots are created every day 22.00. If you have 40@- You don't specify the exact time, when the snapshot creation should happen, but you do specify the retention time for snapshots and the snapshot name. This scenario could be example for databases. You need to have consistent snapshot, and netapp filer doesen't know when the db is consistent. There should be some kind script created in db server which triggers the snapshot creation when the db is consistent..