The National Labor Relations Board announced another deadline extension to file comments on its proposal to change the board’s standard for determining when multiple companies share joint liability for labor law violations.

The board’s September proposal would define when more than one company is a “joint employer” of a group of workers for purposes of labor law liability. It would also identify which entities are responsible for bargaining with the workers if they join a union.

The 30-day extension will push the due date for comments to Jan. 14, the agency announced Dec. 10. The board had previously granted a...