(a) As used in this section and in Sections 51.450-51.454 of this code, “net revenue” means income or increment which may come from ownership and operation of the improvements which are encumbered less the proportion of the district’s revenue income reasonably required to provide for administration, efficient operation, and adequate maintenance of the district’s services and facilities which are encumbered. Net revenue does not include money derived from taxation.
(b) A district which expects net revenue from operations may secure its bonds in any one of the following:
(1) as provided in § 51.433 of this code;
(2) by entering into a contract to pledge the net revenue of the district and to mortgage and encumber part or all of the property and facilities, franchise, revenue and income from operations, and everything acquired or to be acquired by the district; or
(3) as provided in both Subdivisions (1) and (2) of this subsection.