When creating a subscription, the user can decide whether he wants to be notified right away (That may be on the next cron run, depending on system load) or every some time interval (Hourly, daily, weekly).

Send intervals and digesting format for each of them can be configured by the site administrator on Administer > Messaging & Notifications > Notifications settings > Intervals.

For deferred notifications (Send interval different of 'Inmediate'), the messages will be automatically digested, so we'll only get one message (mail, sms...) for each period of time (hour, day, week...). These messages will have a special format, currently consisting of a list of events that have happened for each object.

Example: Short digest for node events

My story (node title):
- The story has been updated by user 1....
- A comment has been posted by user 2...
- ......

Deduping

There may be also more than one subscription producing multiple notifications for the same event. I.e. if you are subscribed to 'story updates' and subscribed to the specific thready 'my story 1', then updating 'my story 1' will actually produce two notifications, one for each subscription.
These go through some deduping process before being sent out, merging notifications for the same event into one notification. However, this can be done if (and only if) both subscriptions are using the same sending method and the same sending interval. Otherwise you'll be getting two or more different notifications for the same event.

Digest templates

For digested messages, different templates will be used. See Administer>Message templates

Events will be grouped depending on event type. Node posts will be grouped by content type, while node updates or comments will be grouped for each node.

Common parts for al digested messages will be taken from Notifications digest template: Subject, Header, Footer...

Then, for each group of eventsr, either 'Group digests per node' or 'Group digests per node type' will be used.

For short digests, on which each event is a text line, it will be taken from the Digest line on the proper template. I.e. 'Notifications for node creation', 'Notifications for node updates', etc...