What is a Sliding Mullion on a Bifold Door?
A sliding mullion is only used on an even number of doors which all fold/slide in the same direction, e.g. a 2 door set or 4 door set. A sliding mullion is not required when the even number of doors are split, e.g. a 4 door set where 1x door goes to the left and 3x doors go to the right.
A sliding mullion is not beneficial but a necessity. It's best avoided if possible as it will increase the sight lines, essentially it is required to prevent the door trolleys/guides from fouling on the jambs (side frames) when the doors are closed.
You can see a sliding/floating mullion in the video here: