An Activity represents a single screen with a user interface(UI). An Android App may have more than one Activity, for example, An email App can have one activity to list all the emails, another activity to show email contents, yet another activity to compose new email. All the activities in an App work together to create perfect user experience.
Activity recognition is the detection of a user's physical activity in order to perform certain actions on the device, such as taking points when a drive is detected, turn wifi off when a phone is still, or putting the ring volume to max when the user is walking.
I recently had to modify the Activity sub-grid to remove certain activity types from the add activity menu.
Note, this may not be a supported method on how to do this, but there is no documented supported way to so it, so I had to come up with a solution & this worked, in CRM 2013 anyway.
An NSUserActivity object can be used to coordinate significant events in an app with the system. It is the basis for Handoff between different devices running iOS and macOS. Additionally, it may also be used to improve public-indexing and augment or create Spotlight Search results for an app. As of iOS 10, it may also be used to coordinate interactions between your app and Siri using SiriKit.