"Supplementary feeding" refers to providing extra nourishment or food in addition to the regular diet, usually to enhance nutrition or fill any nutritional gaps. Full definition
Because of this, it's recommended that you continue to pump from both breasts after each feed to maintain good milk production and have extra stored for supplementary feeds. (charlottekeating.com)
You will need a reduced volume if you are providing supplementary feeding only. (messybeast.com)
If a litter is too large for the queen to support and as long as the kittens are gaining some milk from their mother, they may only need supplementary feeding. (cats.org.uk)