This comes from https://skyline.ms/announcements/home/support/thread.view?rowId=35283
The idea is to make it easy to populate annotations during transition list import. We do have the Note column but it's not very fine grained.
This will probably also trigger the need for a redesign of the transition list import column picker, which is already getting to be ungainly. I'm imagining moving from a simple checkbox list to a dialog subdivided into various panels for precursor, fragment, annotations etc.
This might also eventually be the place where users could define column header synonyms.