How are methylation data normalized?

Hi there,
I am interested in using some methylation data, for example: https://depmap.org/portal/download/all/?release=CCLE+2019&file=CCLE_RRBS_TSS1kb_20181022.txt.gz

The values are normalized between 0 and 1. Is there a documentation showing how the normalization was done? I couldn’t find it in the publication or the website.

Thank you for your help.
Muxin Gu