The "Immediate" notification settings does not send out the notification immediately but "batches" all changes made in the past 'n' mins and sends out a single email.
The "Batch" option on the other hand, will send out a single notification including all changes made in the last 30mins.
It would help to clarify this difference, as "Immediate" might be a bit confusing, as it still sends out "batched" notifications.
We could also add a sample of each type to help make it more clear.
This is with reference to this improvement request implemented in 2.5:
http://jira.atlassian.com/browse/CRUC-137

The notifications have been documented here.