The outstanding bonds of the district shall not at any one time exceed 15 percent of the assessed value of the real and personal property of the district, except where the board elects to use the county assessor’s tax roll pursuant to Article 6 (commencing with Section 6780) of Chapter 7 of this part, in which case the outstanding bonds of the district may equal an amount not to exceed 20 percent of the assessed value of the real and personal property of the district.

Within the meaning of this section the term “bonds” means only bonds payable from the proceeds of taxes levied upon taxable property in the

district.