The game uses a set of custom cards unique to that game in its main mechanics or as a major part of its system, or in much rarer cases uses a non-standard, non-Tarot deck of commercially available cards (e.g. Jass, Hanafuda) as part of its mechanics. This mechanic also covers games that use cards from other games, e.g. UNO or Candyland cards. (In addition, there is a geeklist for this particular subset: RPGs that use non-standard game cards.)
Cards (Specialized) does not include games that have custom cards only for tracking purposes (e.g. to track conditions on characters, to track damage taken, to track magic items received), nor does it include games using index cards for arranging information on the table (e.g. Microscope, Dialect - see the geeklist Card placement games for these). However, if the index cards are used to built a custom deck for shuffling and drawing/dealing random cards, then the game should be linked to the Cards (Specialized) mechanic.