Purpose
By defining
budgeting rules, you can reduce the available budget of budget structure elements, or lock (freeze) the budget for a specific period of time.This process describes how you can define budgeting rules for budget structure elements with FTE budgets. These budgeting rules reduce the budget provided by FTEs in the overall budget. For this reason, you can only define budgeting rules for FTE budgets (and not for monetary budgets).

Your overall budget contains a budget structure element with a budget of 1 FTE. This budget is to be used to finance the position FI administrator, which is staffed by Ms. Smith. However, Ms. Smith is going to retire on June 30 this year. When Ms. Smith retires, the position she occupied is to become obsolete. Consequently, the budget that financed it can be discontinued. By defining a budgeting rule you can freeze (lock) the budget as of July 1. This will mean that the budget cannot be allocated after this time.
Since only customer-specific budgeting rules are defined in Customizing, you should be aware of the following:
When a budgeting rule that locks a budget comes into effect, the budget in question cannot be used for as long as the budgeting rule is in effect. If, on the other hand, a budgeting rule that deletes a budget comes into effect, the budget in question can never be made available again. This means that, when the overall budget is carried forward to the new financial year, it will be reduced by this amount.
Prerequisites
You must have made the following settings in Customizing for HR Funds and Position Management:
When the budgeting rule is defined, the status of the overall budget must be Proposed, Released, Released provisionally, or Being revised.
Process Flow

In the system, "defining" a budgeting rule is referred to as "creating" a budgeting rule.
Budgeting rules mentioned in the official documents that accompany budgets are regarded as binding, and are referred to as external budgeting rules in HR Funds and Position Management.
Any budgeting rules that you define during budget execution are referred to as internal budgeting rules.
See also:
Defining Budgeting Rules Activation of Budgeting Rules Editing of Budgeting Rules