Is the balance tree in CircuitAccount a binary sparse merkle tree?

[Boqian-Ma]

From the implementation, it seems like the balance tree uses the same sparse merkle tree data structure as the circuit account tree.

pub struct CircuitAccount<E: RescueEngine> {
    pub subtree: SparseMerkleTree<Balance<E>, E::Fr, RescueHasher<E>>,
    pub nonce: E::Fr,
    pub pub_key_hash: E::Fr,
    pub address: E::Fr,
}

However, as noted in the illustration in protocol.md, the balance tree is none binary.

What is the correct interpretation?

[Boqian-Ma]

Follow up questions:

1. why does CircuitAccountTree have a sub tree, but not AccountTree?
2. Do both reside in the "server"?
3. Why are there two trees?

[ssutingh]

Great!