In order to encourage developments which are not economically homogeneous and to achieve rent charges which will make units available to persons and families of low income at low rentals, the Authority and a mortgagor may use devices including, but not limited to: direct rental assistance in the form of partial rent subsidy from any county, municipal, State or federal source; allocation of lower rents to less desirable locations and apartments with less expensive facilities; and the raising of rents in the majority of apartments in the development in order to lower the rents of those in the lower rent charge category. With respect to each development the Authority shall, prior to initial occupancy, allocate and prescribe the number of lower rental units and the rents to be charged therefor. The allocation may be reviewed and adjusted from time to time. The method of achieving lower rental charges shall, in each instance, be prescribed by the Authority.