From what people have been digging in the benchmark, Chain seems to follow onto an unknown new effect/ability which seems to trigger something under an unknown specific condition (it's something found notably in the combo finishers like apex -> blast, etc). It's there for a lot of raid buff abilities like DT Radiant Finale as well, etc. Hard to tell how it's gonna be in concrete terms but this is what intrigued me the most actually. Seems like raid buff abilities are getting fleshed out in a way or another.

Succor, Adlo and Emergency Tactics also seem to get something similar. Succor seems to be especially convoluted and goes further than this.