The "known" is the original calibration of the instrument
Purpose:
To separate the concepts of "calibration" from "control checks"
To clarify that "calibration" is something that happens at the factory, not at time of use by a qualified technician
To clarify that control tests at time of testing are simply a verification (to use the CFS scientist's words) that calibration hasn't changed over time (note Hodgson concept of "over time" in his paper referred to in St-Onge)