I ran into the same issue a while back and could not find any information anywhere it was so weird. Plus, is for extended support of the old engine. for example 4.27 was the last build for 4 series and now they have moved on to 5 but there are still a lot for games in the pipeline using 4.27 so they are still supporting 4.27 with builds called 4.27 Plus, for how long they will do this I am not forsure.