If certain requirements are fulfilled in a system with given input variables u and output variables v and all input variables are uniquely allocated to the output variables, this unique allocation is known as the transfer function of the system.
Important concepts regarding the transfer function of a system are defined and described in DIN 19 226 and DIN 19 229. As the transfer function of the individual system variables as a function of the time is allocated to another, it describes the dynamic systems, generally mathematically. How these systems react to this is shown in the dynamic behaviour.