The Previous Action Items card is unique in that it references items that were created in previous meetings within a recurring meeting series.
You can interact with these items (edit, assign, mark as complete, etc.) but you cannot add new items to this card, since these items technically belong to other meetings. That is why we distinguish Previous Action Items from other cards with a yellow background.
Card title vs card type
To understand how the Previous Action Items card works, you need to understand the difference between a card title and a card type. You can change the title of any card, but you cannot change it's type:
The Previous Action Items card displays items from any card with the type "Action Item", even if the card title has been changed to, say, Marketing Team, Product Team and Sales. As long as those cards's type is "Action Item", their content will appear in the next meeting inside the Previous Action Items card.
Likewise, if you add a Note card or a New List card to a meeting and title it "Action Items", the content in those cards will NOT appear in the Previous Action Items card in your next meeting.