A good example would be when I was budgeting for my wedding my static budget was $12,000. I broke down the cost to roughly $4,000 for the venue $4,000 for food, and $2,000 for alcohol, and $2,000 for decorations and miscellaneous costs before I had my RSVPs. Realistically alcohol and food were really controlled by head count though so a flexible budget would be much better to compare to. For example I would be paying $30 a head. so $4,000 was a bit erroneous. I had 108 attendees so $30x108=$3240 then I would compare my final cost to that to see if I was under or over budget.