Answer:This is something you’ll see in the Last Action line for a bill’s status. We don’t write that summary at GovTrack: it’s what Congress reports itself over at the website. But they don’t explain it there either. Since the question was general, beyond the bill where the question was posted, I collected the answers other visitors provided and also asked my expert friends, and here’s what I found out.
Explanation:
Southern slaveholders did not want the Congress to take the power over, so slave trade while controlling foreign affairs. So they only let the Congress have power over trade, excluding slave trade, for next twenty years.