Function account_master_key 

pub fn account_master_key(
    seed: &[u8],
    network: Network,
    account_no: Option<u32>,
) -> Result<Xpriv, DefaultSignerError>

The Spark account master (base): m/8797555'/{account}'. Every wallet key derives from it (identity at 0', leaf signing under 1', static deposit under 3'). account_no falls back to a per-network default when unset.