When having the following use case:
We have 2 words "penslot" and "knoppen". One of them presents a compound word ("penslot"), the other one is a plural form of knop.
When using the compound word filter, if we place the words "pen" "slot" and "knop" in the dictionary, for a search containing "knoppen", we get results containing "pen" also, which shouldn't be the case, because "knoppen" is only a plural form (not a compound word).
We need another dictionary to specify the words that are exceptions to the filter (like in this case "knoppen"). In this case, the filter would find compound words containing "pen" "slot" and "knop", but will leave out dividing "knoppen" and searching on its parts.