In plain terms
The upload endpoint answers with an acceptance. That tells you the file arrived and is queued. It does not tell you the rows inside it passed. Record level validation happens after that, and its outcome comes back separately, by email. So there is a window, sometimes hours and sometimes longer, in which a cycle looks finished and is not. If a row is rejected, the cycle is not filed. It needs correction, and the correction is another upload.
What it means for a registered broker
Keep the two states apart everywhere you track this, in your tooling and in what you tell your own management. Collapsing them is how a broker comes to believe a cycle closed while the state holds a rejection against it. Watch for the confirmation, and chase it if it has not arrived after a few days rather than assuming. Make sure the address it goes to is one somebody reads. And never measure the interval to your next cycle from the submitted timestamp when the filed one is what you would have to show.