Tex. Spec. Dist. Local Laws Code § 9051.257

District bonds may be secured by a pledge of all or part of the net revenue of the district, or by the net revenue of one or more contracts made before or after the issuance of the bonds, or other revenue in the manner specified by board resolution. The pledge may reserve the right, under conditions specified by the pledge, to issue additional bonds that will be on a parity with or subordinate to the bonds then being issued.