Fixed fee projects bill out an agreed-upon fixed amount regardless of the time or expenses spent on the project. By flagging entries as billable or non-billable, you are able to track the actual amount spent on a project rather than just what you billed to the client. In Core, billable entries are entries that are chargeable to the client, while non-billable entries are not charged to the client, but they are still charged against the project. Tracking billable time is important to track the profitability and costs of all projects. Otherwise your business analytics will not be accurate.