The Pro Arté strings are D'Addario's highest end classical nylon guitar strings. They are used by beginners and professionals alike, due to their impressive durability and tonal capabilities.
The different tensions are designed for different uses. For example, flamenco players tend to use the harder tensions as they have a more percussive sound and less sustain. The lower tension strings are more designed for classical playing, however it's best to just try different things and see what feels comfortable.