[Wien] jellium background of the charged supercell

[yxl@email.jlu.edu.cn]

Dear users,
  I have a question about charged cell, Why the electrons can only be removed from the total number of electrons(in case.in2)? Will you please tell me if my comprehension is right. my comprehension : suppose the charge of the supercell distribute homogeneously, and the charge bachground is jellium background.
  I'll be sincerely appreciate your answer.
                                            yours sincerely,
                                                  hongxia