Inhibits activity of the coagulation protease factor Xa in the presence of PROZ, calcium and phospholipids. Also inhibits factor XIa in the absence of cofactors. Interacts with PROZ. Expressed by the liver and secreted in plasma. Phosphorylated by FAM20C in the extracellular medium. Heparin acts as an important cofactor, producing 20 to 100-fold accelerations of SERPINA10 reactions with factor Xa and factor XIa. Belongs to the serpin family.