When a card is created, it defaults to being visible to everyone in your workspace.
You can switch the visibility to limit it to the specific people you want to share the card with by selecting "Visible to invitees." You can also make visible only to you by switching to "Visible to invitees" and not sharing it with anyone.
Visibility only controls who can see and comment on a card. It does not allow editing the contents of the card.
If you want to share a card with your entire team, choose "Visible to workspace". This is great for documents and discussions that you want everyone to see, like your company's Paid Time Off policy and company handbook. These would be able to be viewed and commented on by everyone, but only people who have had the cards shared would be able to edit them.
If you want only a specific set of people to be able to see a card, choose "Visible to invitees" and choose the specific people to share the card with. This is great for private documents like hiring documents and private discussions between small groups of team members.