Reinforced Iron Plate is the first item you encounter in Satisfactory, requiring two items to make it. This means, unlike the Iron Plates, Iron Rods, and Screws that you were producing for the prior milestones via Constructor, you need the Assembler for the first time to automate its production.

Reinforced Iron Plates are needed to build machines like Constructors and Assemblers, but they’ll be used most for Mk.2 Conveyor Belts. So, the way to go is to automate their production as soon as possible.