Set if the activity should be considered as an alternative action to the data the user is currently viewing. See also Intent.CategorySelectedAlternative for an alternative action that applies to the selection in a list of items.
Supporting this category means that you would like your activity to be displayed in the set of alternative things the user can do, usually as part of the current activity's options menu. You will usually want to include a specific label in the <intent-filter> of this action describing to the user what it does.
The action of IntentFilter with this category is important in that it describes the specific action the target will perform. This generally should not be a generic action (such as Intent.ActionView, but rather a specific name such as "com.android.camera.action.CROP. Only one alternative of any particular action will be shown to the user, so using a specific action like this makes sure that your alternative will be displayed while also allowing other applications to provide their own overrides of that particular action.