There are many different ways in which groups and or companies can be paid for their services when they are contracted to perform a particular service and or provide delivery of a final product or products. In some instances, in which the organization and or company (which will be referred to as the vendor from here on) is one with whom the purchasing party has a long established relationship, and one in which they have comfortably worked without any real incident, the vendor may be comfortable providing delivery of the final product without receiving any total of payment in advance. In other cases, where there is a similar level of comfort being extended by the company in question, they may pay in full in advance for the product or service in question. However, in many cases, the concept of retainage comes into play. Retainage refers specifically to the idea of a portion of the contract payment to be withheld pending completion and delivery of the final product.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.