There are five reasons why Projector cannot bring multiple engagements onto the same invoice. If any of these are true you will have to use separate invoices.
if multiple currencies are used in your installation and one billable engagement is in USD and another one is in EUR Projector is going to create 2 separate invoices, one for each engagement/currency.
In order to keep financial data organized if multiple companies are involved a separate invoice will be created for each company and related engagements/projects.
Just like with companies, a separate invoice will be created for each cost center pulled into the invoice request.
If multiple engagements are being invoiced and each engagement has a unique invoicing culture a separate invoice will be created for each culture.
Separate invoices are created for each client. In addition, separate invoices are created for the same client if you are using client levels. For example, if parent client Ford and sub-level client Ford Trucks are being invoiced at the Ford level a separate invoice will be created for Ford and Ford Trucks.