Supplementary things are added to something in order to improve it.
例句
...the question of whether or not we need to take supplementary vitamins...是否需要补充维生素的问题Provide them with additional background or with supplementary information.给他们提供额外的背景资料或补充信息。
英汉词典释义
adj.
增补的; 补充的
There is a supplementary water supply in case the rain supply fails.万一主水源断了, 我们另外有供水的地方。