Hi @tokuryoo!
The assumption on subnets is that strictly less than 1/3rd nodes are faulty, or in other words, f = floor((n-1)/3). Let’s as a concrete example use n=13, then f = 4.
Anything that is signed with “high threshold” requires n-f signature shares, which for our example of n=13, f=4 means 9 nodes need to sign, so strictly more than 2/3rds.
Anything that is signed with “low threshold” requires strictly more than f signatures, so again with n=13 and f=4, it means 5 nodes need to sign, i.e. strictly more than 1/3rd.
You are correct that certifications, CUPs, notarizations and finalizations use high threshold, and random beacon and random tape use low threshold.