S - Planning

S - Planning ( 小 -  計画, Ko - Keikaku) is the excavation process of Klaxosaur carcasses from beneath the Earth’s surface. The Klaxosaur remains dug up are then used to build the FRANXX units.

Overview
After the Klaxosaurs began attacking Magma Energy plants in 2037, scientists from APE, specifically Dr. FRANXX, uncovered preserved Klaxosaur carcasses and began using them to build the prototype of the FRANXX units as a defense arsenal against the Klaxosaurs. Once this experiment proved to be a success, more carcasses began to be dug up from underneath Earth’s surface to build more FRANXX units.

Only those within the APE council knew the truth behind S - Planning and all the children were kept in the dark until they learned the truth from 001 in Episode 20.