AH21 is a basic address lookup module based upon the input of a postcode (or part of a postcode) and an optional premise name or number. Without a postcode (even a part postcode) input, this module will not return any sensible data for you.
ListAPI offers a more generalised address search mechanism. This allows postcode and optional premise lookup, as above, but in addition also gives the capability of looking up on what we term partial addresses (possibly without an input postcode). Where the search criteria is vague or ambiguous, ListAPI has in-built access to the Hopewiser gazetteers providing a series of refinement step results in expandable nodes. This makes it possible (with user intervention) to match to a single delivery point despite the vague or ambiguous input given initially.