does tuple store subtransaction id in it?

In XidInMVCCSnapshot, it will check xid from tuple if is in snapshot->subxip. It means tuple store subtransaction?

But in PushTransaction, I see TransactionState.subTransaction will assign currentSubTransactionId,
currentSubTransactionIdwill reinitialize in StartTransaction. So I think tuple should not store subtransaction id. 

I am confuse about this. If subtransaction id will reinitialize every start time. How to judge it is a subtransaction
fromxid in tuple?