The Budget Overview functionality allows uses to see in real time the Remaining Available Budget and the ability to drill down into individual transactions.  

Real time means once a transaction is successfully budget checked, the information is updated.  The structure is based on the budget keys.  

The DETAIL Ledger Group uses non-translated (detailed) values for Expenses.  The DETAIL Ledger Group Remaining Balance could be negative since the ledger is set to Track without Budget.

The APPROP Ledger Group is the legislative Control budget for Expenses.  This is based on budget key roll ups for Account; Department.

The REVENUE Ledger Group is for non translated (detailed values for Revenues).