Merge End is used for Thin Ribs (when profile is open curve and thick option is checked) and Merge Rib's End option trims off extremity of rib to existing material.
Merge End Option: The Merge Ends option is to be used for thin ribs (or slots). It trims a set of profiles to themselves while trimming them to existing material too.
The green curves is a sketch for Profile and Red for Center Curve, Thick option is checked for this example, its must for merge option.
Merge rib's ends Option : The Merge rib' ends option trims each extremity of the rib to existing material, starting from the profile position until the first limit encountered on existing material in each direction.
In this file, green is used for Profile and Red is Center Curve.
If Merge Rib End option is chekced the profile (a green circle) swept along the red center curve is trimmed on each direction to the first faces encountered.
If Merge rib's end option is not selected, you will get following output