EmitVertex();
tells OpenGL that what have finished to set vertex attibutes values (here only the position) and that a new vertex can be generated (okay in our case it does not generate a new vertex, since vertices already exist).

I’m not quite sure what you mean to say, but maybe it should be:
“This tells OpenGL that (after geometry shader processing,) vertex positions will be set. Also that new vertices can be generated, (even though the example shown here generates no new vertices.)

I’m still learning about geometry shaders, so I take no responsibility for the technical accuracy of my attempted edit, only for the grammar.

Thanks for sharing your experience with us!

vertex
(
position(x,y,z) = 3 floats * 4 bytes,
color(r,g,b) = 3 bytes;
color(alpha) = 1 byte – which side of cube is visible using bitfield (one bit per side, 0 = not visible, 1 = visible)
)

Then in geometry shader – take one primitive and generate a colored voxel cube of max 6 sides and min 1 side depending on precomputed bitfield value(color-alpha byte), with normals for each side for light?

And it is possible to store generated triangles into a VBO?