For polarized standard stars the pipeline determines the difference between the measured linear polarization (Lobs) and the value tabulated in the catalogue (Lexpct). 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. The difference is then normalized by dividing through the tabulated value.
The reference value (0.5) currently corresponds to the median of the measured values.