The categories (or Carousels) can be added to the playlists to show the contents associated with them.
When the player executes the sequence of the playlist, at the moment of executing the carousel, it will only execute one video each time it executes the sequence of the playlist. When the playlist sequence ends and start again, the second video will be played in its order and so on until the last video.
Look in this animation, a visual explanation of how the carousel behaves:
To know how to add a carousel to a playlist, we recommend the article How to Associate Content to my Playlists