The implicit answer is given in a Note in the tutorial on mobility calculations, see
https://docs.quantumwise.com/tutorials/mobility/,
"The sampling ranges are chosen so that only the relevant regions of the Brillouin zone in k-space and q-space are sampled"
To better understand this note, you should go through the theory section and read the reference paper cited in the tutorial, doi:10.1103/PhysRevB.93.035414. I notice that your question is not related to QuantumATK software as such, but rather to a general scientific topic on the electron-phonon scattering. This is why it is important to understand the theory to be able to do the actual numerical calculations in QuantumATK.
In the case of graphene: "For the former this region is that in the vicinity of the K-point, which is located at [1.7063, 0.0, 0.0]. For the latter, this region is that in the vicinity of the Γ-point at [0.0, 0.0, 0.0]", as given in that Note for k and q area in the Brillouin zone. These area are related to the choice of the electron states (->k area of the BZ in the electron band structure) and phonon modes (->q area of the BZ in the phonon band structure) that are supposed to mainly contribute to the e-ph scattering events. So, you have to look at the electron and phonon band structure of the material, as well as the position of the Fermi level. For graphene, the Fermi level crosses the K-point, and the area around that point is where the e-ph scattering is expected to take place.
I note that you can, in principle, sample the entire BZ, but then your calculation will unnecessary become too expensive computationally.
