SOS is a Morse code distress signal, used internationally, that was originally established for maritime use. In formal notation SOS is written with an overscore line, to indicate that the Morse code equivalents for the individual letters of "SOS" are transmitted as an unbroken sequence of three dots / three dashes / three dots, with no spaces between the letters.

In this case, it’s being designated for what seems like a separation of one European country from, well, being able to claim that they are part of Europe. Which may actually just bring them more independence.