TIME & MATERIAL
At times scope, specification and implementation plans of an engagement are not easy to define and specify at the outset. For such projects the effort estimation for all the elements and aspects of the project and for all stages is usually not possible right at the beginning.
In such situation resource utilization based billing is done. There is a specific rate associated with each resource depending on the experience, skill, role etc. of the resource in the project. The effort is charged on this pre-agreed rate (hourly, weekly, monthly) until the project is delivered in its totality.