H1 Extension : What does this status mean?

I have recently applied for my first H1 extension and currently seeing below status in USCIS website. Its very vague, has anyone received similar status? Not sure what it means

We mailed your document for your Form I-129, Petition for a Nonimmigrant Worker, Receipt Number WAC********. If you do not receive your document by September 4, 2015, please go to www.uscis.gov/e-request to request that we resend the document to you. If you move, go to www.uscis.gov/addresschange to give us your new mailing address.

Also here is my timeline

7/28 - Application received by USCIS for regular processing
8/3 - Request forwared to USCIS by employer to convert to premium
8/5 - Status on USCIS changed from “Application received” to above vague status (“Documents mailed”)