In this review titled "Pieces of the Flavour Puzzle" the author Ferruccio Feruglio wrote in the introduction that
"The origin of the parameters in the flavour sector of the Standard Model (SM), minimally extended to include massive neutrinos, is one of the most enigmatic questions in particle physics. Out of the 22 (20 if B-L is conserved) independent low-energy parameters, which with some abuse of language can be called Yukawa couplings, 18 have been measured. Of the remaining four parameters, the absolute scale of neutrino masses is constrained in a limited range, the leptonic Dirac CP-violating phase starts to be constrained by global fits while the two possible Majorana phases are still unknown."
I know what are neutrino mass-squared differences. I also know that an upper limit on the sum of the light neutrino masses come from Planck observations. But what is meant by the absolute scale of the neutrino mass?