Explain restricted assignment problem

In free or unrestricted assignment problem all jobs can be performed by all individuals or all machines. But, in practice, it is possible that a particular worker is    not competent perform a particular job. In such situation that job can not be             assigned to that corresponding worker. This is known as restricted assignment problem. To handle this problem of restriction on job assignment correspond cell ‘– ——‘ (dash) is consider and no algebraic steps are perform at this cell. In optimal solution that combination of assignment is not possible.