The current value (leaf page) timestamp and transaction ID ordering (in structures, function calls and in on-disk cells), is
start_ts, stop_ts, start_txn, stop_txn
and the address (internal page) timestamp and transaction ID naming ordering is
oldest_start_ts, newest_durable_ts, newest_stop_ts, oldest_start_txn, newest_stop_txn
This order wasn't chosen for any particular reason, it was a result of an incremental implementation.
Alexander Gorrod and I would like to switch to
start_ts, start_txn, stop_ts, stop_txn
for values, and to
newest_durable_ts, oldest_start_ts, oldest_start_txn, newest_stop_ts, newest_stop_txn
This is 4.1 required because it can't change once it's in the field.