Xero Projects is an awesome product by Xero, which enables to keep track of time and costs for each project. Among other features, it allows to easily link Bill line items to specific projects and keep track of project expenses.
We'd love to show you which projects and Bills have been linked, and to provide you with a way to route Bills depending on the project via our approval matrices. Unfortunately, this is not possible at the moment because the Xero Projects API does not allow it.
We're in touch with the Xero team regarding this issue and hope that the Xero Projects API will be extended in the future.