A Link (リンク, Rinku) is a term used in Genso Suikoden Card Stories. Links are what allow successive character cards to be played in the game.
Overview
There are nine link groups in the game, named in alphabetical order from A to I. Each character card (a card from the Leader, Commoner, Craftman or Free types) has one or more of these links. Once a Leader card is played, only a card with at least one matching link is able to be played with it. This continues on, with the following card needed to match at least one of the second cards links and so on.
Example
The Leader card McDohl has links in groups A, B, E and G. Once played to lead a Mission or Facilities card, the next character card must have one of those links.
You may wish to follow up with the card Yuber but its links are C and H, neither of which are shared with McDohl, so it cannot be played. The Free card Mina, however, has links in E, H and I. Thus, it can be played following the McDohl card, as they share the E link.
As Mina has links in E, H and I you can now play the Yuber card following it, as they share the H link, even though Yuber shares no links with the original McDohl card. If you wished to follow Yuber with the card Lorelai, you could not as even though it shares the A link with McDohl and the E link with Mina, the next card must be played off of the card preceding it, in this case Yuber's C and H links. So, in this case, you would have to play the card Zaj, which shares the C link. From there, a card with the C or I link must be played.
In this way, your range of options can be controlled and expanded or restricted depending on what cards you use to take advantage of the links available. This continues until the Mission or Facilities card is complete. Each Mission or Facilities card is a unique chain with no mixing or dependency on other chains elsewhere on the field.