Difference "as determining meaning or as being safely excludable from the determination of meaning"

No repetition is exact, but the meaning of a sign depends on taking it as the exact repetition of some other sign. Nonetheless, the meaning of a sign, as linguists have told us, lies not in its exact contours but in the possibility of differentiating it from other signs, adjacent or nearby, in the possibility of recognizing that an "a," however mad, is an "a" and not a "b" or a "z." In any sign something is always left over that is not sublimated in its meaning but remains stubbornly heterogeneous, unique, material. . . . At the same time this exigency makes all texts undecidable in meaning. They are undecidable because the role of that physical substratum either as determining meaning or as being safely excludable from the determination of meaning, as trivial or accidental, can never finally be decided or sure. Does it matter, for example, that blue, black, or red ink is used to inscribe a given written document? It might or it might not. No convention or code can ever fully circumscribe these alternatives. Each letter mark, or sign, as Jacques Derrida has more than once said, must have an ideal iterability in order to be identifiable and have meaning. At the same time each mark is divisible, marked by the possibility of being used, in whole or in part, in different contexts and therefore with different meanings. Derrida names this propensity to wander away from itself, intrinsic to any sign, "destinerrance."

Miller, J. Hillis. Ariadne's Thread: Story Lines. New Haven: Yale U.P., 1992. print. 8-9, emphasis added.