Backend activity and notification edit will now give details regarding their assets. It will give you a full breakdown of the asset, show you the context it believes belongs to the asset, and the class responsible for its source object. In addition to this it lets you know the template paths to customize the asset for adjusting how it renders or overriding access, source, and context as desired.
Additional backend explanations of asset are on the way as well. This should significantly improve your understanding of how asset works making custom activity and notifications even easier to implement. In most cases it's as simple as making a single template file and on that note there is now examples of how to implement custom asset sources, contexts, and access checks in addition to custom activity and notification displays. These examples are included as example.php and contain several comment blocks explaining how they work and showing how they work.