Complex sentence: They are closely related to the Racah W-coefficients, which are used for recoupling 3 angular momenta, although Wigner 6-j symbols have higher symmetry and therefore provide a more efficient means of storing the recoupling coefficients.
Split into simpler sentences: They are closely related to the Racah W-coefficients, which are used for recoupling 3 angular momenta. They have higher symmetry than the Racah W-coefficients and therefore provide an efficient means of storing these recoupling coefficients.