A budget is a committed deviation from the forecast

A forecast is a projection of past spending into the future. A budget is a deliberate, committed behavior change that decreases expected spending to increase savings.

For years, I built budgets with a vague sense of historical actuals informing a hand wavy sense of the future. Every “budget” was really a guess dressed up as a plan, and we were often off for expected reasons. Now the forecast is transaction level certainty that tells us what happens if nothing changes, and the budget records exactly which lines we chose to change and why.

Actuals get compared to both. Drift is life not going as expected. Discipline is us not holding the line. Different problems, different fixes.