Rustic and primitive. Two common terms in furniture. The easiest way to really break it down is to think of a Venn Diagram. Rustic is one circle, Primitive the other.
Primitive is where the material itself dictates the form and is not enhanced extensively by hands. It is how you present it that creates the piece. It is almost like a piece of art, a sculpture. And that is its function, although it can also be a bench, per se. The original form remains but it blends into the piece.
A builder can get away with a lot of the sins of the original piece of wood, while utilizing more of the wood.
Both really demonstrate good technique and craftsmanship, especially when the purpose and functionality of the piece of furniture comes together seamlessly.
A gentle touch to maintain the look. Primitive has an old, simple look and utilizes early techniques like butterfly keys and carving. So do rustic.
Both styles are an evolution of furniture, just as Queen Anne, Duncan Phyfe, Mission, Stickley, Louis XIV and XV. They are all built according to a line. A good piece has a horizontal plane in space.
It is a modern extension of the vanity of furniture. There really are no limits; it just needs to be solid.
But it is a challenge to figure how to create a nice smooth horizontal plane. A plane attached to a piece of furniture that is not machine cookie cut. They become heirlooms then.