The DOS is number of electrons/ energy, i.e. unit 1/eV. If you integrate it over the energy to the fermi level, you get the total number of electrons in you system.
For a DeviceConfiguration we only calculate the DOS for interior atoms, i.e. if you have 20 atoms in the central region, and you have 4 in the left electrode and 6 in the right electrode, we do only calculate dos for atom number 4-13 in the central region
for technical reasons we cannot calculate the dos for atom 0-3 and 14-19,