The assets are basically self explanatory.
following = include activity you are following
following.only = only include the activity you are following
connections = include your connections activity
connections.only = only include your connections activity
self = include your activity
self.connections = include your activity and your connections activity
self.connections.only = only include your activity
self.following = include your activity and your following activity
self.following.only = only include your following activity.
self = viewing user
user = displayed OR viewing user
displayed = displayed user
This combination I found: "user.connections,following", what is suggested for using in which cases?
Suggestion is to not touch asset unless you specifically need a custom activity stream. Vast majority of users will just be using profile activity and for that case just leave it empty. I do not and have no plans to provide a giant list of all the possible usecases as they're effectively unlimited, sorry.
For example lets make up a scenario. You've a 3rd party Joomla extension that provides an event calendar. You want an activity stream for that event calendar. You could have an asset of events.EVENT_ID for a stream specific to that event id.