Daigo fan-site and general Japanese fighting game blog, Beast Note, has posted their translation of the System page on the official Under Night In-Birth website.
There is a detailed breakdown of the EXS and GRD (Grind Grid) meters and how they work together. The EXS meter is a general use meter for supers, EX moves and other things, while the GRD meter is unique in that it is based on the momentum between players.
It is interesting to note that Beast Note mentions that there are no comeback mechanics in this game. Get the rest of the info at the link below.