Plates that are normalized separately

In the paper “Plasma proteomic associations with genetics and health in the UK Biobank” by Sun et. al, in the discovery cohort (n = 34,557), they only “included participants of European ancestry from batches 0–6, excluding the plates that were normalized separately, and batch 7 (COVID-19 imaging longitudinal samples and baseline samples showing increased variability and mixed with COVID-19 imaging samples)”

I wonder how to identify the plates that were normalized separately, and why they are normalized separately.