According to GASB codification Section 1600.106, the following excerpt attempts to explain the “availability” concept.
.106 Revenues and other governmental fund financial resource increments (for example, bond issue proceeds) are recognized in the accounting period in which they become susceptible to accrual—that is, when they become both measurable and availableto finance expenditures of the fiscal period. “Available” means collectible within the current period or soon enough thereafter to be used to pay liabilities of the current period. Application of the “susceptibility to accrual” criterion requires judgment, consideration of the materiality of the item in question, and due regard for the practicality of accrual, as well as consistency in application. The length of time used to define available for purposes of revenue recognition in the governmental fund financial statements should be disclosed in the summary of significant accounting policies.