C3S seasonal forecast graphical product - details

Hi, I was wondering where can I find information on the seasonal forecast graphical products?

For example, this plume plot for IOD for JMA for May Start: https://climate.copernicus.eu/charts/c3s_seasonal/c3s_seasonal_plume_rjtd?facets=Parameters,SST%3BCentres,JMA&time=2022050100,0,2022050100&type=plume&area=iod

Which start dates/ensemble members are used for this plot? The latest JMA seasonal prediction system has 5 ensemble members per day for forecasts. Does this plot use all members for forecasts and hindcasts?

Also, there is a note beneath the graphical products:

Ensemble members
The solid lines show the forecast anomalies from all the individual forecasts; the blue dotted line shows the respective recent observations. The anomalies are re-scaled to ensure comparable total variance for the individual models, in the monthly time scale. The variance standardization is based on 1993-2016 (with the exception of NCEP, where 1999-2016 is used for NINO regions). For the IOD, anomalies with respect to the 1993-2016 climate are shown. For NINO indices, anomalies with respect to the 1981-2010 climate are shown.

The variance standardization mentioned here is only for the multimodel ensemble? Where can I find more information on variance standardization employed in these plots?