I know I've seen at least one site that has used a notation like this to represent some arbitrary turn of a particular layer. For example, Ux might mean turn the U layer (clockwise, counterclockwise, or half-turn - pick one). If Ux appears again in the alg, it means to do the same U layer turn. If Ux' appears in the same alg, it means to do the inverse at that U layer turn.
For example, to orient 2 edges in th U layer, you could write: (I will use n instead of x to avoid confusion with x cube rotation.)
R' E' R2 E2 R' Un R E2 R2 E R Un'
Depending on which U layer turn is performed for "Un", this flips either UB, UF, or UL along with flipping UR. It's a sort of shortcut way of writing three similar algs as a single alg.
I agree with hait2 that Ux normally would mean the same as U x (x being a cube rotation). This alternate use of x (or y) by some site(s) I see as somewhat unfortunate as it conflicts with the more standardized meaning of those letters.