I assume they use N⋅D rather than ND to make it explicit these are 2 different variables. That's not necessary for 6N because variable names don't start with a number by convention.
Well no actually it'd still clear to me that they mean the the multiplication of 3 different variables X, N, and D.
I don't think of it as eliding obvious operators. Rather in mathematics juxtaposition is used as an operator to represent multiplication. You would never elide an addition operator.
So X next to D still means multiplication as long as you can tell that X and D are separate entities.
I would wonder why they switched conventions in the middle of an expression though.
To clarify, if it read:
C ~ X N⋅D
you'd be as confused as me? Its because its a number it has special implied mechanics where we can skip operators because its "obvious".