BTL means better than life and it's a form of Simsense chip that basically has all the knobs turned to eleven and no filters at all. It allows you to experience everything recorded at much more intensity than normal Simsense chips. There are a couple of popular varieties, from porn (of course) to what is claimed to be snuff to recordings that feature only emotions. Another variety are personafix chips - slot one of those and you believe you are another person. A popular use of this variety is in prostitution.
BTL is highly addictive and illegal in many parts of the world.