(a) In this section, “net revenue” means the district’s gross revenue and income from all sources less the amount necessary to pay the cost of maintaining and operating the district and the district’s property.
(b) Bonds issued under this subchapter may be secured under board resolution by a pledge of:
(1) all or part of the district’s net revenue;
(2) the net revenue of one or more contracts made before or after the issuance of the bonds; or
(3) other revenue or income specified by board resolution or in the trust indenture.

(c) The pledge may reserve the right to issue additional bonds on a parity with, or subordinate to, the bonds being issued, subject to conditions specified by the pledge.