The upper and lower energy levels of the transition. The syntax depends on molecule:
For linear molecules without spin (most cases), the number given is J.
For linear molecules with a triplet Sigma ground state (e.g., SO), the numbers given are N and J.
For symmetric top molecules, the numbers given are J and K, while for ammonia, the symmetry of the inversion part of the wave function is also given.
For asymmetric rotors, the numbers given are J,K_p and K_o.