Under "Bamboo administration >> Security >> User repositories" Bamboo can manage its own users and groups or obtain them from another source.

1. Local users and groups
   Users and groups are stored and managed in Bamboo.
2. Users and groups from JIRA or Crowd
   Users and groups are stored and retrieved from a JIRA or Crowd server.
3. Custom user repository
   Users and groups are stored and retrieved from an LDAP server or a custom repository as defined in the atlassian-user.xml file in the Bamboo home directory. Refer to the documentation on integrating Bamboo with LDAP for more information.

When setting up option 3 from above, by going to "Plan configuration >> Notifications >> Add notification" you can notice an icon aside "User" input field

Whereas, when option 1 or 2 is selected, the icon does not get displayed.