The ESS linac has experienced several changes in the architecture and the latest modification was performed to meet the ESS cost objectives. This process reduced the final energy of the linac to 2.0 GeV. The third iteration of the 2.0 GeV linac, the Optimus, has been developed to respect the vacuum and installation requirements as well as the new cryomodule design while keeping the impact on beam physics to a minimum. This report summarizes the design rules applied to this linac.