MatchMerger merges the matches in the same request based on the .id attribute. The matches with parents of the same id will be merged into one single Document's matches
Assuming you have 20 shards and use top-k=10, you will get 200 results in the merger.
When adding an Executor to the Flow, MatchMerger is used in the uses_after attribute so that the executor returns one Document with 200 matches instead of 10 Documents.
MatchMerger is also used when you want to combine the matches retrieved from different pathways. Assuming you have two paralell retrieval pathways, one pathway retrieves 10 matches based on images and the other retrieves 10 matches based on audio. When combining the requests from the two pathways, you will receive two Documents with the same id but matches from different pathways. You can use MatchMerger to merge the matches into one Document with 20 matches.