Distinguish fixed, variable, and discretionary expenses

Expenses can be classified by the mathematical pattern and degree of control they exhibit: fixed expenses remain approximately constant over a budgeting period, variable expenses change with usage, quantity, or prices, and discretionary expenses can be reduced, postponed, or omitted by choice. These categories are not mutually exclusive—for example, a discretionary subscription may be fixed—so accurate budgets classify each cost by both its behavior and necessity, supporting totals, comparisons, and projections.