The Unit field is one of the crucial elements when creating a budget.
Services, the integral parts of every budget, can have units set as:
Note that tracking time on a service is available for all types uf units, whereas expenses can only be tracked on services set up as 'Pieces'.
You can also disable time and expense tracking per service. To learn more, check out the following article: Time Tracking and Expense Tracking options.
Here is a brief overview of how units, budgets and tracking options are related:
Hour / Expense