That would be wherever they’re most needed; though it could be argued that if the Central Powers haven’t achieved their objectives before the Americans start arriving in Europe in significant numbers then they’re unlikely ever to do so. US entry is the ticking clock the CPs have to beat, though as I’ve suggested its start could be linked to Russian collapse rather than a mandatory turn 4.
Therefore plugging the gaps in Allied lines, notably stopping western capitals falling, is the most usual use for US imports.