A standard stream records a position in a table's change history. Querying it shows changes since that position. A SELECT does not consume them. A successful write that reads the stream advances its position, so a later consumer sees only newer changes. Each independent consumer needs its own stream.
The result includes METADATA$ACTION, METADATA$ISUPDATE and a row identifier. Updates appear as a delete and an insert, while a row inserted and removed before consumption can disappear from the net change set. The simulator computes changes against the stream's stored table snapshot. It advances the offset only after the consuming write succeeds. There is no background scheduler or external source. You control each change and consumption statement in the worksheet, which makes a failed or repeated load visible.