Tax exemption and pension opt out

Hi there! I was wondering whether the tax exemption should appear in your work contract (the contract between the researcher and the institution)? e.g. should it say that you are going to pay taxes according to xy treaty or be more specific and say that you are going to pay taxes in a certain country? Also, the USS Pension "Opt Out" should be added to your contract or one needs to address this question later on? Thanks! Annamaria