A measurement is a set of response data uniquely identified by parameter type or equation, response type, port numbers and project file. An example would be:
DB[S21] - steps.son
DB identifies the response type as magnitude in dB. S identifies the parameter type as an S-Parameter. 21 identifies the input port as Port 1 and the output port as Port 2. The project file from which the measurement originated is "steps.son".
Measurements are assigned to a curve in the Manage Curves dialog box in the Graph tab.