1. A control system comprising: a master station that includes a first processing unit that executes a user program; and a remote station that includes a first buffer to which a device accessed by the first processing unit in accordance with the user program is assigned and that is used for data transfer between the remote station and the master station, a second processing unit, and a subunit to or from which one or more pieces of data is input or output, the remote station being connected to the master station through a transmission path, wherein the first processing unit transmits, to the second processing unit, first designation information designating an object that is among the one or more pieces of data and to which a device assigned to the first buffer is assigned and second designation information designating an assignment rule, and after the first designation information and the second designation information are received, the second processing unit assigns data designated by the first designation information among the one or more pieces of data to the first buffer in accordance with the assignment rule designated by the second designation information.