Usually - the initiator of the Deviation Request - gives the complete details about the intended change. The change proposed may be discussed between the engineering, site and project execution departments to estimate the time and financial implications of the change, it may also under go an approval to the site head level, before being released to the customer.
The customers team again will review the request for deviation and associated time and commercial implication - before deciding on the next course of action. Deviation requests may also lead to a scope change order under which the scope of the work of the contractor may change and may have cost and time implications / requirement of additional compensation.
Usually - each project may have different formats for the deviation request forms, a sample of the deviation request form can be found here.