"Endothelial Cell Growth Supplement (ECGS) is a medium supplement designed for the optimal growth of human primary microvascular endothelial cells in vitro. It is a sterile, concentrated (100X) solution which contains growth factors, hormones, and proteins necessary for the culture of normal human microvascular endothelial cells. The supplement is formulated (quantitatively and qualitatively) to provide a defined and optimally balanced growth environment that maximally promotes the growth of human primary microvascular endothelial cells in vitro. The supplement is designed as an additive for endothelial cell medium (ECM, Cat. No. 1001) and should be used in conjunction with that medium." https://www.sciencellonline.com/endothelial-cell-growth-supp...

"The Endothelial Cell Growth Supplement is designed for use as a cell culture media supplement."
https://www.emdmillipore.com/US/en/product/Endothelial-Cell-...

"The Endothelial Cell Growth Supplement is designed for use as a cell culture media supplement."
https://ru.vwr.com/store/product/826085/endothelial-cell-gro...

Not sure the layman's "complex" (usually applies to multivitamin/mineral supplements) is applicable here.