Two categories of photoelectric standards are
distinguished:
[][
C:] the star is a photoelectric standard with confirmed
long-term stability (with three or more independent measurements over
several years);
[ S:] the star is a secondary photoelectric standard star
(based on two independent measurements).
[]The photometric accuracies required for a programme star to be
considered as a photometric standard range from 0.020 to 0.026 mag
(standard error depending on the 
magnitude, Grenon 1989b). The photoelectric standard stars are used to
calibrate the magnitudes derived from the detection chains of the main
mission (Hipparcos magnitude, ) and of the Tycho experiment
(
and 
magnitudes), and for monitoring the detector responses
(including their colour dependence), the linearity of these responses,
and their evolution throughout the mission (Grenon 1985b, 1989b). In
the final version of the Hipparcos Input Catalogue, there are 10822 C
stars and 3068 S stars.