How does this differ from the normal superseding return that has existed for some years?
A normal superseding return is one that is filed after an original return, but is still filed by the original due date (or the extended due date if the original return was extended).
So, if partnership doesn’t request an extension, and files its ’18 return on 3/15/19, there is no chance for a valid superseding return. Now, if the partnership had extended it’s return (and still filed it’s original return on 3/15/19), then the partnership could file a superseding return all the way up to the extended deadline.
The guidance that Makbo cites deal with certain partnerships that filed by 3/15/19, but who didn’t request an extension. They normally cannot file a superseding return…but the relief in the Rev Proc says, “We’ll let you file a superseding return this one time, even though you don’t meet the requirements.”