When should a time and material contract with a contractor be chosen instead of a fixed$-$price contract?

When project scope is not well known at the beginning of the project and scope knowledge evolves as the project progresses.

When cost risks for the customer should be limited but not schedule risks.

When subcontractors are being used and the customer will reimburse their invoices.

When only a limited amount of money can be made available to fund the project.