You can link the cards in a deck with specific cards in your collection. This is helpful for tracking where your cards are located and knowing which cards you need to acquire to complete the deck.
To link a card in a deck, go to the Details page for the deck and find the card you want to link.
Hover over that card with your mouse, then click the button one in from the bottom right corner. "X/Y Links" will appear when you hover over the button, where X is the amount of cards already linked and Y is the total number of cards in the group.
A window will open and all the cards in your collection that match will be listed. From that list, find the card from your collection that you want link and click the link icon in the leftmost column of the list.
The link icon will be grey if the card is not linked to any cards in any decks. It will be black if the card is linked to a card in the same deck. It will be red if the card is linked to a card in a different deck.
If you own the card already but haven't added it to your collection yet, you can click the 'Add Card' button. A copy will be added and listed, ready for you to link.