The opening cap is used as an alternative to a Monkey's tail at the start of the stairs or an ornate start to straight runs of handrail, the opening cap allows for a change in direction on a continuous handrail that has newel posts at the change of direction.
With traditional opening caps the straight section of handrail is cut in with a "V" shape at the correct angles for the profile from the straight handrail to the round opening cap to intersect with the profile meeting.

3D model of the handrail termination: opening cap or Bun.

Knostairs 3D
Knostairs 3D model

Opening Cap Gallery.

Opening cap being restored.
Opening Cap Metal core rail