Qiskit function phase_amplitude_damping_error

I am reading about the noise models that can be simulated in Qiskit and I found out the phase_amplitude_damping_error function. I read about it and it seems to be a function that simulates a combined dephasing and generalized amplitude damping channel (amplitude damping with finite temperature). I am pretty sure that the function implements such channel. However, I am wondering where do the Kraus operators that are described there come from (it is a Krauss rank 6 channel), as I have not been able to find any bibliography regarding such combined channel.