There are various cube shapes in Robocraft. These are:
You can attach other cubes on all 6 sides. However, this is the heaviest cube shape (talking about chassis), so inners can fully replace the cubes, as they are 1.5x lighter and have same attaching capabilities. The only difference is with LCC/LCI, where LCC is for free and LCI costs 3 .
You can attach cubes anywhere, like on a full cube, but an inner is lighter. See Full cube.
You can attach cubes on 4 sides on a prism, and it's only half a cube (or 2/3 of an inner) in terms of weight. It's recommended to build a chassis from prisms where you can and from inners where not.
The tetra has only 3 sides you can attach to and it's weight is a third of a cube's weight, a half of an inner's weight and two thirds of a prism's weight.
All movement, hardware, special and cosmetic items have irregular shapes and most (but not all) have only one attachable side - square 1×1 (the one they are usually attached to chassis by). Weapons and thrusters are the biggest. For example, T10 Plasma Launcher is 7×7×6 cubes (6 cubes being the height).