Core Units define ownership and administration in a Construct 
From the Kickstarter comments:
"At the moment, what makes possible any construct to be identified is its Core Unit. Currently, the core unit gives an ID to the ship, but is also the Element from which you can make a blueprint of the construct, as well as managing the ownership rights of the construct."
From KickStarter Update 14:
Size of your construct will be governed by the core unit.
"The size of constructs will be limited only by your ability to gather the resources to build them, plus the max building zone size of the Core Unit you are using to create it. We plan on having very large Core Units available later in game, and the ability to assemble several constructs into one by “anchoring” them (this may not be available at release time however, and could be in a -free- expansion)."
There are at least two different types of core units.
"Constructs can be either static (from a Static Core Unit) or dynamic (from a Dynamic Core Unit). Only dynamic constructs can move, so this is mostly for spaceships. Static Core Units have several advantages that will make them necessary. Like the potential to host protection bubbles vs simple shields. We are considering the possibility of “anchoring” a dynamic construct at some point to give it some of the benefits of a static construct, but this is not decided yet."