As far as I see, Intl.Collator() and localeCompare() have locales argument. Mainly it helps to specify BCP 47 language tag, but also this property can indicate such parameters as co, kn, kf.
I've read in MDN about co that...
Variant collations for certain locales. Possible values include: "big5han", "dict", "direct", "ducet", "gb2312", "phonebk", "phonetic", "pinyin", "reformed", "searchjl", "stroke", "trad", "unihan". This option can be also be set through the options property "collation".
I don't understand what exactly co does. What does it collate? Maybe it concerns collating the locales when we specify one more language tag?