I don't know what the other term for it, since this is the first time I heard and on what category it is included. I attached again screenshot so you can cleary see what it is look like.
As you can see in the picture, the secondary duct of customer has a “Shoe Connector”. They said that it is the normal design on HVAC which I believe one great factor when they explain it. The Air Flow start from left to right, the angle slopes purpose (the left of shoe) is to have a proper entry of air flow to the branch duct.
Hope you can help me with this duct connector family.