Pretty much as above. Part 145 requires an agreement or contract to be in force; in the case of the one-off, the fact that an AMO requests another entity to perform maintenance is considered an agreement, with a fax or email to back it up. This "one-off" could have been between the engineer as an ICAO licensed engineer "in the desert" or between Thomas Cook and Thomson's respective Part 145 AMOs. ["One-offs go between Part 145 AMOs or Part 145 AMO/LAE and are not issued by operators as such (other than if they are approved under Part 145) but the operator has to be kept in the loop]