What is the difference between project budget and plan in SAP Projects?

Project budgets impose a strict constraint to expenditure enforceable via budget availability control settings. Project budgets may be constrained overall or per fiscal year. Budgets may be distributed down to lower-level WBS elements to control expenditure at a granular level. Changes to project budgets are controlled by differentiating the original budget from supplements and returns. Project plans represent the planned expenditure. Project plans are typically less restrictive than budgets, although as of S/4HANA, planning versions can also be used to enforce availability control. As opposed to project budgets which are annual, project plans can be maintained by month. Planning can be done overall or at individual cost element level. Changes to plan expenditure are handled by using different planning versions.