The "kWave_BreastCT.mat" is an example of Breast phantom data from a k-Wave simulation. For more details, please refer to the "k-Wave Simulations" section in this link.
"d_obs_240x240_1mm_0p3MHz_new_512.npz" is the output of the code "breast_simulation.py", which is a observation data of the sound speed model.