Sometimes things happen that force you to stop a process. In a case like this, you want to block the related card, so that everyone knows the status of the card and the reason why it’s blocked. Only licensed users (administrators, collaborators) can perform the blocking/unblocking.
More settings about card blocking can be found on Card Blocking Settings.
To access the Card Blocking Settings, go to Board Settings on the top/right side (only possible if you’re an administrator). Then, select Card Blocking from the list of settings.
Here, you can prevent blocked cards from moving on the board and prevent blocked cards from affecting WIP limit.
Open the card that you want to block and click on the Block icon (that’s on the right side of the card).
On the ‘Block Reason’ dialog box, type in the reason why you want to block the card then click the ‘Block This Card’ button.
Once a card is blocked, it will show a Block icon, indicating the reason, as well as who blocked it and when. The block icon can also be seen from the board.
Open the blocked card that you want to unblock and click on the Unblock This Card button. Now, the block is removed and you can continue working on the card.