"The common EU format of having a blue section on the extreme left with EU circle of stars and the country code was introduced by Council Regulation (EC) No 2411/98 of 3 November 1998[1] and entered into force on the 11 November 1998."
https://en.wikipedia.org/wiki/Vehicle_registration_plates_of...
Maybe not all countries adopt the EU standard, but it makes sense that there is an "eu" mode.