For each user, you can decide which notifications they should receive:
Emails for tasks: the user receives an email every day if he has 'only due' tasks, 'none' or 'always'
Calendar entries for events: the user receives a ics calendar entry for 'all', 'own' or 'none' events
Emails for limits: the user gets emails as soon as a limit (max number of subscribers, ...) is reached for 'all', 'own' or 'none' events. If none, one checks whether at least one user receives the email. If not, the account administrator will receive the email even if he has set 'none'.
'Own' events are events where the user is registered with 'Events-> Contacts-> Manager' as manager. This option is only available if you have selected under Account-> Default view-> All.