Whenever a detailed specification cannot be provided at the start of a project, or if the Agile Development Methodology or the Spiral Model should be used, then we offer the T&M contractual model.
With this model, we bill the performed work days (and other services like special hardware or software licenses) on a monthly basis.
The T&M model is preferred, when the number and skills of required resources may change in the course of a project.
The difference to the Dedicated Team model therefore is the planned variation of resources.