Default notification sound / Notification ringtone
This option allows you to set the default ringtone to be played when you receive a text message or other notification for which you have enabled sounds, such as an incoming email.
It should be noted that this setting only defines your default notification ringtone, and that individual apps that use notification may override the default with their own tone. Many apps allow you to choose your own notification ringtone for use in that specific app, letting you identify the app that wants you attention from the ringtone alone without needing to pick up or interact with your device.
Tones are chosen in the same manner as in the Phone ringtone option above.