All relative kinematic quantities between frame_a and frame_b are defined, i.e., relative position, relative velocity and relative acceleration (resolved in frame_a).
For efficiency reasons, these calculations are performed in subclasses (= the same equations are just solved for different variables according to the usually needed causality). The relative quantities, which are defined in this model, have the following meaning:
S_rel : Rotation matrix relating frame_a and frame_b, i.e. if
hb is vector h resoved in frame_b and ha is vector h resolved
in frame_a, hb = S_rel*ha.
r_rela: Vector from the origin of frame_a to the origin of frame_b,
resolved in frame_a.
v_rela: (Translational) velocity of frame_b with respect to frame_a,
resolved in frame_a: v_rela = der(r_rela)
w_rela: Angular velocity of frame_b with respect to frame_a,
resolved in frame_a: w_rela = vec( der(S_rel)'*S_rel )
a_rela: (Translational) acceleration of frame_b with respect to
frame_a, resolved in frame_a: a_rela = der( v_rela )
z_rela: Angular acceleration of frame_b with respect to frame_a,
resolved in frame_a: z_rela = der( w_rela )
If needed, all of the above quantities can also easily be resolved in frame_b, according to Xrelb = S_rel*Xrela . However note, that v_relb is not der(r_relb) (v_relb=S_rel*v_rela; r_relb=S_rel*r_rela).