TeachMeFinance.com - explain unobligated balances
unobligated balances -- The portion of budget authority that has not yet been obligated.
When budget authority is provided for one fiscal year, any unobligated balances at the end of that year
expire and are no longer available for obligation. When budget authority is provided for a specific
number of years, any unobligated balances are carried forward and are available for obligation during
the years specified. When budget authority is provided for an unspecified number of years, the unobligated
balances are carried forward indefinitely, until either they are rescinded, the purpose for which they
were provided is accomplished, or no disbursements have been made for two consecutive years.