Two modes to duplicate meshes along a curve.
First one is the default one, which deform the mesh around curvy path. useful for lace or soft objects.
The second one is rigid duplicate, which does not deform the mesh. useful for hard object such as bike chains and tank treads
After each operation, in the “redo last” panel, you can adjust the mesh size, distance between duplicates and offset along the curve. on the 3D view side panel, you can also adjust the necessary property later.