Used on various signs to indicate that an area should not be entered, e.g. a military installation, or a hotel room.
Used on a road sign to indicate a one-way street that cannot be entered from the direction one is facing when seeing the sign.
Source: en.wiktionary.orgDo not enter if you have traveled to an area that is under a Level 2, 3, or 4 travel advisory by the U.S. State Department. Source: Internet