How to say "supplementary" in Japanese

• 副ふくCommon
  noun, prefixReally a prefix for a secondary position or role — the 'vice-', 'sub-', 'associate' and 'assistant' senses. It is not the adjective to use for 'supplementary' meaning extra or additional; for that, 補助的 or 補足的.
• 補助的ほじょてき
  na-adjectiveThe 'auxiliary; subsidiary; ancillary' sense of 'supplementary': something that supports or assists a main thing — extra income, staff, or equipment backing it up — rather than filling a gap in it.
• 補足的ほそくてき
  na-adjectiveThe 'complementary' sense: material added to fill in what is missing and complete the whole — a supplementary explanation or extra details, for instance.

As an adjective, pick 補助的 for something that supports or assists a main thing and 補足的 for something that fills in what is missing; 副 is the 'vice-/sub-/associate' prefix for a secondary position.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 副ふくassistant
• 補助的ほじょてきauxiliary
• 補足的ほそくてきcomplementary