User blog comment:BertH/New on-site notifications/@comment-27702369-20170328143602/@comment-5275700-20170328150555

Answer to both questions: As time goes on, more things besides discussions can potentially have notifications delivered via the new system. Even things that may not be communication-related. So it makes sense to use the bell for that system. The older system will only ever be delivering Wall and Forum notifications, so it gets the text bubble.