Championship and Challenger skin clarification

So, I'm aware that for the Victorious skin Riot has said it's given to the champion that created an impact during the competitive season, but what defines who gets the Championship/Challenger skins?

For references: S3 championship S4 championship S5 championship

S4 'challenger'

It just hasn't been announced or made public afaik and I'm just curious, they're somewhat attached to Victorious considering their competitive themes and whatnot.