Because slacklining is still a young sport and there are still few materials and standards that are geared to the slackline application, material from other applications (from sports and industrial climbing to industrial applications) is used.
Here in this article it is shown that this is carabiners can be used in a few applications while slacklining. Be aware that you never use aluminum carabiners in your rig. They tend to break under cyclid load.
This video shows how the carabiner shape affects the breaking load of triangular loads.