Any modification or change to works agreed in the contract is treated as a variation.
Depending on the magnitude of the change, it may or may not alter the original contract amount and/or completion date.
After the original scope (or contract) is formed, complete with the total price to be paid and the specific work to be completed, a client may decide that the original plans do not best represent his or her definition for the finished project.
Common causes for change orders to be created are: A project manager then typically generates a change order that describes the new work to be done (or not done in some cases), and the price to be paid for this new work.
Once this change order is submitted and approved it generally serves to alter the original contract such that the change order now becomes part of the contract.