Function store_boundary_snapshot 

async fn store_boundary_snapshot<T>(
    storage: &T,
    height: u64,
    state: &ValidatedState,
) -> Result<()>
where
    T: SequencerStateDataSource,
    for<'a> T::Transaction<'a>: SequencerStateUpdate,

Store state, the state at the epoch boundary height, as the loop’s head snapshot. The loop keeps nothing of state: on every start it rebuilds the head’s state from the header and fills it from this database, so the head needs what a state update stores:

• The reward tree. The loop reads it from here only, so it goes first: a head without it would fail every start.
• The block frontier. Without it, the loop would fetch it from peers again, and block proofs at the head would fail.
• The chain config. The loop takes it from the header, but only the loop stores it, and chain config catchup reads it from here.

No state is stored below the boundary, so the state pruned height goes just below it: queries for lower heights then answer that the state is pruned, as on a node that pruned it. The boundary is also recorded as where the state starts, so that a node meant to have complete trees can tell that it does not.