Function start 

pub fn start(
    protocol_version: u64,
    reference_gas_price: u64,
    start_timestamp_ms: u64,
    start_checkpoint: Option<u64>,
    system_state_bcs: Option<Vec<u8>>,
) -> Value

Build a partial StoredEpoch carrying the start-of-epoch fields. Indexer pipelines that observe an epoch’s first checkpoint stage this as a merge operand against the epoch’s key.

start_checkpoint is itself optional: the embedded fullnode seeds the current epoch’s row from a mid-epoch SuiSystemState at restore time and cannot know the epoch’s first checkpoint (it precedes the restore tip). Passing None leaves the field unset, and because the merge operator only copies fields that are present, a later full start record from the upward backfill fills it in without either operand clobbering the other.