A Senate Action Subcommittee was a limited committee of the Galactic Senate established temporarily to deal with specific, limited issues.
During the Clone Wars, a number of new action subcommittees were formed, each overseeing the Galactic Republic's direction of the war effort for various regions and sectors of the galaxy.
The fall of the planet Duro to the Confederacy of Independent Systems in 20 BBY, and the revelation that the Senate Action Subcommittee for Corellian Trade Spine Defense's corrupt chairman had reassigned ships from Duro to Iseno for a bribe, resulted in public outcry against the Senate. It lead for a push to restructure—and possibly dismantle—the war action subcommittees, with their powers being assigned to the Supreme Chancellor. Senators Lexi Dio and Padmé Amidala opposed this move, stating that it would place limits on the self-governance of sectors and stifle the voice of the Senate.
The Sector Governance Decree presumably abolished the war action committees and gave their power to the Moffs.