supplementary
UK: ˌsʌplɪˈmɛnt(ə)ri | US: ˌsʌpləˈmɛnteri
adj. providing additional support or information
adj. completing or enhancing something when added to it
n. (supplementaries) something that completes or enhances another thing
The word "supplementary" traces back to the Latin supplementum, which combines sub- ("under") + plere ("to fill"). The concept of "filling up what is lacking" evolved into the modern sense of "adding to complete or enhance." The suffix -ary was later attached to form an adjective, solidifying its role in describing something that provides additional support or completion.
The teacher provided supplementary materials to help students understand the complex topic.
This vitamin is a good supplementary source of nutrients for your diet.
The report includes a supplementary appendix with detailed data.
She took a supplementary course to improve her skills.
The supplementary income from freelancing allowed him to save more money.