Peter A Goldsbury
有 yuu / 有る aru means to possess, have, or be.
無 mu is a negative particle.
有段者: yuudansha: someone/those possessing a dan rank
無段者: mudansha: someone/those without a dan rank
有級者: yuukyuusha: someone/those possessing a kyu grade
無級者: mukyuusha: someone/those without a kyu grade
級段者: kyuudansha: someone/those with kyuu grades and/or dan ranks. In this case 有 is missing. A similar case would be 高段者 koudansha: someone / those with high dan ranks.
I think you forgot the term used for special cases such as mine . . .
rank, some of us
would have no Aikido rank at all!