If you make a clip persistent, the clip will be 'above decks', so by doing this, you remove the original clip from the source deck - but it is still available in all decks, as it is persistent.
If you then make the clip unpersistent in a different deck, then the clip will be placed to that deck only, as a regular clip.
The primary purpose of the persistent clips could be for example a Logo layer on top of your composition, where you would have many clips set to persistent, so you don't have to copy the clips to other decks on change.
The small thumbnails represent the clips that are on the deck you switched to, "below" the persistent clip.
In the original deck, there is no clip any more in that clip slot.
Kadete wrote:video is overrided as expected on that layer. but sound keeps playing.
Which clip overrides which one, which clip's sound track is playing?
The persistent clip should always be the one that launches and plays both audio and video in a shared clip slot.