The Assist Modules feature unlocks as a milestone reward in Tier 19, Wave 40. Players can spend stones to unlock an Assist Module slot for each module type. When an Assist Module slot is unlocked, you can equip a module that matches that slot type.
Assist Modules maintain their Main Effect, Unique Effect, and Substats at a reduced efficiency. You can increase the efficiencies of these values with stones and through the Assist Module lab research (unlocked at Tier 19, Wave 50). You will not be able to equip modules that share the same name.