Artificial staple fibers in their raw, unprocessed form are classified in the heading for artificial staple fibers not carded or combed. Once they have been carded, combed, or otherwise mechanically processed to align fibers for spinning, they move to 5507. The distinction turns on whether the fiber has undergone that preparatory textile processing step.

Yes. The heading does not subdivide by fiber type at the 6-digit level; 550700 covers all artificial staple fibers that have been carded, combed, or otherwise processed for spinning, regardless of whether the base fiber is viscose rayon, lyocell, modal, or another artificial variety.