Regarding FIDIC Yellow and Silver Books (edition 1999) the Contractor has submitted its design proposal...

The Contractor is entitled to change the design by optimising the design, without approval of the Employer/Engineer.

The Contractor is not allowed to make any changes regarding the design to optimise the design, unless approved by the Engineer/Employer.

The Contractor may submit a proposal for Value Engineering.

The Contractor is not allowed to submit a proposal for Value Engineering, as any value engineering should already have taken place before submitting its design proposal.