The official help page has provided an example of German language.
https://help.libreoffice.org/6.1/he/text/shared/optionen/01010400.html
In language-dependent custom dictionaries, the field contains a known root word, as a model of affixation of the new word or its usage in compound words. For example, in a German custom dictionary, the new word “Litschi” (lychee) with the model word “Gummi” (gum) will result recognition of “Litschis” (lychees), “Litschibaum” (lychee tree), “Litschifrucht” (lychee fruit) etc.
Can someone provide an english example just to make it clear how it works?