XC (eXtra Cards) cards are supplemental sets to your APBA season card set collection.
This set has a niche audience, and is intended for serious replayers and league use. Many of the players carded, played limited time and aren't household names.
This set is intended for those who want the ultimate realism and use of an entire roster for their replays or leagues. These sets are printed in house on perforated stock, 9 players per sheet. Master Symbols for the XC players are included on the cards.
Beginning with 1995 this set is in card format.
With the expansion of the base set to 900 cards. Many XC sets are now part of the base set for that season. For example the 1956 Master baseball set includes the XCs.