Added in API v1.4. You can specify where in the menu hierarchy your action will be placed. By default, Indigo will create a submenu at the bottom of the Actions list and put your actions there. If you specify “DeviceActions”, Indigo will create a submenu on the Device Actions menu and put your actions there. If you specify “NotificationActions”, Indigo will insert your action n the Notification Actions menu without a submenu (be sure to name your action appropriately so that it's clear what it does).
The reason for this is to make sure that actions that target a specific device, regardless of whether it's a built-in device or a plugin defined device, are available from the Device Actions action type menu. Ultimately we think users will be less confused about device-specific actions if they are there.
The same goes for notifications: if your plugin defines some kind of action whose usage is primarily a notification, then use that option so it shows up on the Notification Actions submenu. We may add more options in the future to help guide users to the right place when looking for the actions they want.
Just a reminder - the next time you do an update to your plugin, just take a quick look to see if using one of these attributes is appropriate.