In my days as a system engineer there were three types of requirements:
1. Those that were mandated and had to be achieved in the manner specified.
2. Those were end result was mandated, but the means of achieving the end result was open to negotiation.
3. Those that could be waived if th end result could still be guaranteed to be achieved.
Signing a contract and then attempting to negotiate away class 1 requirements always lead to substantial heartache for the manufacturer. To quote your contract compliant questioner - which part of mandatory did you not understand?
They can, of course, be changed - at substantial cost to the contractor. After all, they probably underbid the contract assuming they could do it their way regardless of the contract they signed....For example on wiring looms....