The NFLPA has appealed the 10-game suspension of Greg Hardy.
Hardy received a suspension for domestic violence incident involving his former fiance.
The union is expected to claim that Roger Goodell applied the discipline retroactively under the new personal conduct policy, which was not enacted until December - seven months after Hardy was arrested on domestic violence charges.
Goodell, however, has claimed that the suspension is identical to what it would have been under the old policy for conduct detrimental to the league.
Hardy signed this offseason with the Dallas Cowboys.