The notification scheme is a reusable entity that links events with notifications. In other words, it contains the associations between events and their respective e-mail recipients:
From this screen, you can see a list of all the notification schemes and what projects are currently using them.
JIRA comes with a generic default notification scheme. The default scheme is set up with notifications set for all the system events. This allows you to quickly enable notifications in JIRA. The default setup has the following notifications:
Current Assignee
Reporter
All Watchers
You can modify the default notification scheme to add your own notification rules, but as you grow your JIRA adoption, it is a better idea to create a new scheme from scratch or copy the default scheme and make your modifications.