"Action", in the sense of a judicial proceeding, includesrecoupment, counterclaim, set-off, suit in equity, and anyother proceeding in which rights are determined.
For employers who have filed claims that have been denied by the IRS, in some cases, they can request a Form 907 agreement to extend the time to bring suit with the IRS, and in other cases, they are better off filing suit in district court sooner rather than later.