I think this speaks for itself.
Welcome aboard sonny. You will be employed by CX primarily to fly nice shiny new wide body passenger aircraft. However, we also fly freighter aircraft as well and ................ under a working “Freighter Aircraft Crewing Agreement”, passenger crew can agree to fly these aircraft with other passenger crew or with freighter crew (referred to as “mixed crew flying”) if and when required (by the company). If you want this job, and we know you do, your signature at the bottom of this letter represents notification that you are willing to take part in such freighter operations if necessary. However, this will not affect your salary or benefits.