The requirements of the software license that governs the Linux operating system were questioned on Thursday as The SCO Group and Linux advocates offered differing interpretations of the role that the GNU General Public License (GPL) would play in a legal dispute between SCO and IBM.SAN FRANCISCO - The requirements of the software license that governs the Linux operating system were questioned on Thursday as The SCO Group and Linux advocates offered differing interpretations of the role that the GNU General Public License (GPL) would play in a legal dispute between\u00a0SCO\u00a0and\u00a0IBM.In March, SCO launched what has become a $3 billion lawsuit against IBM, claiming the company had improperly added code to Linux. SCO, which owns rights to the original System V Unix software, has since claimed that IBM may no longer distribute its own version of Unix, called AIX, and threatened Linux users with lawsuits.IBM on Wednesday\u00a0filed a complaint\u00a0with the U.S. District Court in Utah charging SCO with a number of counterclaims, including patent violations, breach of contract, and breach of the GPL.SCO responded to the countersuit on Thursday, calling IBM's complaint an effort to distract attention from flaws in its own business model and criticizing the GPL. "It's a fairly nebulous license," said SCO spokesman Blake Stowell.Whatever questions may surround Linux's license, they are compounded by the fact that SCO this week began offering to sell Linux users new software licenses that would bring their Linux systems into compliance with SCO's intellectual property claims. Linux advocates argue that, because it distributes Linux itself, SCO is retroactively tacking a second, conflicting license onto software it has already distributed."They need to realize that they have licensed the software under the GPL and released it to the world, and they have no rights to ask for royalties," said Bradley Kuhn, the executive director of the Free Software Foundation, the organization that created the GPL.SCO stopped selling Linux in May. Stowell admitted that his company was still providing Linux source code and security patches on its Web site in order to fulfill support contracts with customers, but he disputed Kuhn's claim. "If our IP (intellectual property) is being found in Linux and that's being done without our say, then I don't think that the GPL can force us not to collect license fees from someone who may be using our intellectual property," he said.IBM's complaint echoes Kuhn's criticism. SCO has included GPL code in its Linux products and "by so doing, SCO accepted the terms of the GPL," the complaint says. By seeking licensing fees, SCO is in breach of the license, it says.The Free Software Foundation's general counsel, Eben Moglen, is consulting IBM in its counter suit, according to Kuhn.IBM's claim of GPL violations is noteworthy because the GPL has never been tested in court, and the IBM counter suit could ultimately decide whether it is enforceable under U.S. law.Recently, a German legal expert called into question the software license's enforceability under European law.The GPL has not gone to court in the past, Kuhn said, because companies have always settled with the Free Software Foundation, rather than risk a court case. "The GPL doesn't need to go to court to be proven," he said. "The lawyers on the other side don't want to go to court because they can't win."