HIGH    Use high priority notifications primarily for important communication, such as message or chat events with content that is particularly interesting for the user. High priority notifications will get the Heads-Up Notification display starting in L.
DEFAULT    The default priority. Keep all notifications that don't fall into any of the other categories at this priority level.
LOW    Use for notifications that you still want the user to be informed about, but that rate low in urgency. LOW notifications will tend to show up at the bottom of the list, which makes them a good choice for things like pubic/undirected social updates: the user has asked to be notified about them, but they should never take precedence over urgent or direct communication.
MIN    Contextual/background information (e.g. weather information, contextual location information). Minimum priority notifications will not show in the status bar. The user will only discover them when they expand the notification shade.
How to choose an appropriate priority
Default, High, and Max priority are interruptive priority levels and risk interrupting the user from what they are doing. This should not be taken lightly, so these levels should be reserved for notifications that:
    Involve another person
    Are time-sensitive
    Might immediately change the user's behavior in the real world
Notifications set to LOW and MIN can still be very valuable for the user. Many if not most notifications just don't need to command the user's immediate attention, or vibrate the user's wrist, yet contain information that they will find valuable when they choose to look for notifications. Criteria for LOW and MIN priority notifications:
    Don't involve other people
    Aren't time sensitive
    Is content the user might be interested in but could choose to browse at their leisure
 
Set a notification category
If your notification falls into one of the predefined categories (see below), assign it accordingly. Aspects of the system UI such as the notification shade (or any other notification listener) may use this information to make ranking and filtering decisions.

Notification.CATEGORY_CALL    Incoming call (voice or video) or similar synchronous communication request
Notification.CATEGORY_MESSAGE    Incoming direct message (SMS, instant message, etc.)
Notification.CATEGORY_EMAIL    Asynchronous bulk message (email)
Notification.CATEGORY_EVENT    Calendar event
Notification.CATEGORY_PROMO    Promotion or advertisement
Notification.CATEGORY_ALARM    Alarm or timer
Summarize your notifications
If your app creates a notification while another of the same type is still pending, avoid creating an altogether new notification object. Instead, turn it into a summary notification for the app.
A summary notification builds a summary description and allows the user to understand how many notifications of a particular kind are pending.
You can provide more detail about the inpidual notifications that make up a summary by using the expanded digest layout. This allows users to gain a better sense of which notifications are pending and if they are interesting enough to be read in detail within the associated app.
Make notifications optional
Users should always be in control of notifications. Allow the user to disable your app's notifications or change their alert properties, such as alert sound and whether to use vibration, by adding a notification settings item to your application settings.
Use distinct icons
By glancing at the notification area, the user should be able to discern what kinds of notifications are currently pending. Android系统英文文献和中文翻译(3):http://www.youerw.com/fanyi/lunwen_28864.html