The Union core of a "Fleet of Fog" vessel appears as a flattened metallic sphere about three inches in diameter with a number of smaller spheres merged into it. The core also has a "Fleet of Fog" insignia on its top side.
Inside the Union core, there is a powerful quantum computer that is capable of self-awareness, quantum communication devices and conventional communication systems; Kirishima was able to hack into human radio channels even when she lost both her hull and her Mental Model. Union cores are also capable of manipulating unaligned or friendly nanomaterials around them.
Union cores are quite durable, but they are unable to manifest a Wave-Force armor on their own - this requires a construction of needed hardware with nanomaterials. Thus, they can be destroyed with conventional weapons.
- The only "Fleet of Fog" vessel that doesn't seem to have a union core is Maya (anime). Not so sure about does she actually have one in the manga, though.