Card visibility is a setting that allows you to either prioritize or hide cards in certain contexts and views.
This setting can be found under any card's ellipsis (...) menu.
Currently, there are three visibility settings: Priority, Visible, and Invisible.
This is the default. A visible card will generally just show up where you expect.
This is reserved for cards that are important and they will appear in a few more places than is normal. Notably, this is the Outline (the hierarchy of cards on the left of the screen), where only Priority cards are shown.
You might expect all your cards to be added to the Outline in the sidebar automatically. We have avoided doing this as once you have 100s of cards in the sidebar it starts to lose its purpose. Instead only the cards that are important to you should be in your Outline. That's why every-time you open a card (Double-clicking it's title or pressing [shift] + [space] when it is focused) and make it a parent we automatically change it's visibility to 'Priority' so it is added to the card tree permanently.
This setting allows you to hide a card from most views. However, it is not the same as deleting a card, so all you need to do to see invisible cards is disable the Invisible filter in the filters panel. Invisible cards with never show up in the Command Prompt's global search feature.