To select multiple issues, you can do either of the following:

• Selecting multiple ungrouped cards (i.e. cards that are not adjacent to each other) — Hold down 'CTRL' on your keyboard and click to select the desired cards.
• Selecting multiple grouped cards (i.e. cards adjacent to each other) — Click a card, hold down 'SHIFT' on your keyboard and click another card above or below the first one. The first card, the second card and all cards in between will be selected.
  Tip: If you hold down 'SHIFT' on your keyboard and click an issue, without clicking a card first, then all issues from the top of the Planning Board down to the clicked issue will be selected.