For unpolarized standard stars the pipeline determines the measured value. Since polarization parameters are usually tabulated for broad-band filters the pipeline selects the broad-band filter closest to the central wavelength of the grism.
Values significantly different from zero might indicate instrumental polarization. Since the error of the polarization plays an important role here we plot it in the lower panel.
Only data with an error below 0.5 in instrumental polarization are trended.

The corresponding instruments polarization normalized with its error used to be monitored. However, since 2024-04 we have been monitoring the actual value of the polarization.