Historical information

The flag was placed on an empty chair, the Siege Perilous, for Court Rituals.
Siege Perilous - Arthurian legends
Some legends also note a special chair at the Round Table, called the Siege Perilous, or 'Dangerous Chair'. Legend says that the seat could only be used by a knight who had a pure heart. A person with a pure heart was seen as someone who had committed no crimes or done any wrong. In the tale, anyone who sat in the chair but did not have a pure heart would die instantly. It was kept free by Merlin for the person who would one day find an object called the Holy Grail. This was a magical cup that was supposed to have been the one that Jesus drank out of during the Last Supper. In the story, this person was either Sir Galahad or Sir Percival, depending on what version of the story is told.
https://simple.wikipedia.org/wiki/Round_Table

Physical description

White flag with MOK symbols in centre of red cross and at each corner.