Supplementary pet food is legally defined as pet food, which has a high content of certain substances, but which in its composition is sufficient for a daily diet, only if it is used in combination with other pet foods ( FEDIAF Nutritional Guidelines for Complete и Complementary Pet Food http://www.fediaf.org/images/FEDIAF_Nutritional_Guidelines_Update_De December_2018.pdf )
Complementary pet food covers a wide range of products, including:
a. Products which significantly contribute to the energy content of the daily ration but are not complete;
b. Products intended to be mixed with other food components in the household to form a complete feed;
c. Treats and snacks are normally given to strengthen the human animal bond and as rewards during training. Although they are not intended to contribute significantly to the daily ration, they may be given in quantities that impact total energy intake.
Regulation (EC) No 767/2009 of the European Parliament and of the Council of 13 July 2009 on the placing on the market and use of feed, Article 3, p. 2 j)