Note Commitment Trees.
A note commitment tree is an incremental Merkle tree of fixed depth used to store note commitments that Action transfers produce. Just as the unspent transaction output set (UTXO set) used in Bitcoin, it is used to express the existence of value and the capability to spend it. However, unlike the UTXO set, it is not the job of this tree to protect against double-spending, as it is append-only.
A root of a note commitment tree is associated with each treestate.