This juxtaposition of letters has a few meanings:

DX: Whacked-out Japanese abbreviation for deluxe. For example: Tetris DX.

Dx (where x is a number): A single die with x sides.

d`x` (note that the d is traditionally typeset in roman): Leibniz's notation for the differential amount of `x`.

δ`x`: The partial derivative of `x`.