In the early 80s, there was some controversy as to whether or not cards from Traded sets were "true" rookies. This was due to their Hobby-exclusive factory set distribution. To some, if you couldn't pull it out of a pack, it's not a "true" rookie card. Because of the widespread distribution and general acceptance of such sets, the XRC designation was discontinued after 1988.
Beckett however, grandfathered all existing XRCs, thus creating a rather confusing situation where a player can have his "rookie card" in a set AFTER his XRC was in a previous year's extended set.
For example: Barry Bonds's 1986 Fleer Update card is his first card and, had it been released just a few years later, would be his true rookie card. As it is, the '86 Fleer Update is considered by Beckett an "XRC," while Bonds's 1987 Fleer is given the "RC" label. General consensus in The Hobby is to treat BOTH cards as rookies.
In 2001, Beckett revived the XRC when dealing with the controversial 2001 Donruss Elite Extra Edition cards.