By interplanetary space, we are talking about an environment in which space radiation is much more intense than it is in LEO, because above Earth orbit is the geomagnetosphere, the Van Allen belts, which trap charged particleradiation coming from deep space.
The AMS is a particle detector that will use radiation from space to help ascertain conditions at the beginning of the universe as well as providing more data about its composition.
The Alpha Magnetic Spectrometer was designed to search for dark matter by collecting high energy radiation known as cosmic rays, and then sort through it looking for a certain class of particle believed to be created after two bits of dark matter collide in space and destroy each other.