Unassigned At Birth (UAB or UAAB), also seen as Gender/Sex Undesignated At Birth (GUAB/SUAB) or Unassigned Gender/Sex At Birth (UGAB/USAB) refers to someone who does not have an assigned gender at birth. It is most commonly designated with the letter "U". It is most commonly used by parents who are raising their children without a prescribed gender, though it may be used for other reasons. It may also be adopted by someone who doesn’t care about or pushes away their "birth sex". UAB children may be described as ante gender.
It is unknown who coined the term. There have been instances where people choose to have their children unassigned at birth. 
The flag appears to have been coined on March 3rd of 2019 by Tumblr user beyond-mogai-pride-flags. It has no confirmed meaning.