The unit's acronym ("SEAL") is derived from their capacity to operate at sea, in the air, and on land – but it is their ability to work underwater that separates SEALs from most other military units in the world. Navy SEALs are trained and have been deployed in a wide variety of missions, including direct action and special reconnaissance operations, unconventional warfare, foreign internal defense, hostage rescue, counter-terrorism and other missions. Without exception, all SEALs are male members of either the United States Navy or the United States Coast Guard.The United States Naval Special Warfare Development Group (NSWDG), commonly known as DEVGRU and informally by its former name SEAL Team Six (ST6), is one of the United States' two secretive Tier One counter-terrorism and Special Mission Units (SMUs).The other such group is 1st SFOD-D aka Delta Force.