Apparently it's an acronym for a type of diving. The letters stand for building, antennae, span and earth. I hold a USPA A license. Usually a walk-on visiting jumper can manifest after showing his or her logbook and U.S. license. Usually, the logbook is more important than the license since it clearly documents your currency as well as your skills and experience.
Some countries require that all jumpers hold medical insurance.
But it's been our experience that the medical-insurance requirement isn't widely enforced, at least not with visiting foreigners. We've never been required to have it. Some countries require that visiting foreigners join their national aero club, but again, we've never been required to do so during our travels. It makes sense to join if you're residing in the country and intend to jump regularly, even if it's not required, as the aero clubs do a lot to keep us in the air. We think you'll find yourself in the air pretty quickly.