From the introduction:
If you played AD&D second edition back in the 1990s, you probably remember kits. These character options were introduced in The Complete Fighter’s Handbook, and became a mainstay of the rest of the books in the Player’s Handbook Rules Supplement series. Though kits were notorious for providing wildly uneven benefits (some offered all-but-unnoticeable improvements, while others turned characters into walking engines of destruction), they did add depth and customization to a game that had previously offered few character choices after 1st level.
This month, Unearthed Arcana converts a few of the more popular kits from that era to new class options for the bard and fighter.