This study will compare WATCHMAN FLXTM ("DEVICE") to a category of OAC called non-vitamin K antagonist oral anticoagulation, also known as NOACs ("CONTROL"). NOAC medications include Pradaxa® (dabigatran), Eliquis® (apixaban), Xarelto® (rivaroxaban), or Savaysa® (edoxaban).The study will research the safety and effectiveness of the WATCHMAN FLXTM device compared to treatment with NOAC therapy to determine if it works for patients with your type of atrial fibrillation who require treatment for potential blood clot formation (thrombus) in the LAA.