When you are signed up for a project, there's a budget estimation that you typically would come up with. During the course of a project you may want to check how you are fairing in terms of the budget that you've set for a project.
In Zoho Invoice, you can set up a budget and monitor how well you are maintaining your budget with the help of a report. A budget can be set for the cost of the entire project, the total hours spent on the project, etc.
Zoho Invoice FAQ