When you use the Japanese or Ainu input source, the following Japanese dictionaries are available to you:
Basic dictionary (基本辞書): The basic dictionary of kana/kanji conversions. It can’t be changed.
User dictionary (ユーザ辞書): Your custom dictionary to which you’ve added words, phrases, or symbols to use for conversions.
The dictionaries can contain Japanese idiomatic expressions, corresponding mixed kana/kanji expressions, and parts of speech.