Purpose
You can use
budgeting rules to decrease the available budget of budget structure elements, or to freeze it for a specific period of time.This process describes how you define budgeting rules for budget structure elements with FTE budgets in the budget hierarchy. Budgeting rules reduce the number of FTEs available 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 specific budget structure element which has a budget of 1 FTE. You use this budget to finance the position of Accounting clerk. The holder of this position is Thor Nielsen. Mr. Nielsen is going to retire on June 30 this year. When he retires, the position will become obsolete, and the budget used to finance it can be saved. You define a budgeting rule that locks the corresponding budget as of July 1. This means that the budget of the budget structure element can no longer be allocated.
Since budgeting rules are entirely user-definable (Customizing), you should read through the following points:
When a budgeting rule that locks FTEs comes into effect, the budget of the budget structure element cannot be used within the validity period specified. When a budgeting rule that removes FTEs entirely comes into effect, the budget is no longer available and cannot be made available again. The original budget is reduced when the overall budget is carried forward to the next financial year.
Prerequisites
You must have made the following settings in Customizing for HR Funds and Position Management:
The overall budget must have the budget status Proposed, Released, Released provisionally, or Being revised when the budgeting rule is defined.
Process Flow

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