In the Acquisitions module, money is encumbered and expended through funds. Funds allow you to set and track spending for areas of your collection. You can define those areas however you like. It can be by collection (e.g. juvenile), by subject area, by format, by branch, by department. It can be a combination of these. It's totaly up to you.
Here are some questions to ask yourself when trying to decide how to organize your funds: