The Holo-Projector is an item added by LibVulpes. It allows players to create a virtual 3D schematic of the structure they are building. Scrolling with the mouse wheel while holding shift when holding the Projector will cycle through block layers or display all at once.
Models that can be viewed by the holo-projector include the:
Astrobody Data Processor
Electric Arc Furnace
Area Gravity Controller
Orbital Laser Drill