Every animation contains an internal skeleton object with bones that used to animate your avatar. (Imagine these bones like the bones of the avatar MannyTheSkeleton - which has build on the top of these bones).
Poser can read any imported animation in the system. Poser reads the bones of the current animation and create the selected bone droplist.
By selecting a bone from the droplist you can modify the rotation, scale, and position of the current bone. The default avatars (John, Jane, and MannyTheSkeleton) have 31 bones with the following order:
The selected bone become the target bone
for the current animation framekey.
(rotation, scale, and position controls target to the selected bone in the current framekey).