Here's a bit from one of the e-mails about the CAS, from PGT fee deposits:
"In your Unconditional offer letter, we asked you to pay a tuition fee deposit of £1000. Before we can apply for your CAS number, we need confirmation that you have paid this deposit.
* If you have paid, please email a copy of the receipt to pgtfeedeposits@sheffield.ac.uk.
* If you are sponsored by a government agency or company and do not need to pay a fee deposit, please email a copy of the letter confirming your sponsorship to pgtfeedeposits@sheffield.ac.uk. Please note that if your sponsor is not recognised by the University of Sheffield (eg a family member), you will still be required to pay the deposit."
For example, if you were paying out of pocket with no form of financial aid at all then you'd be required to pay the fee deposit. But if you're using recognized financial aid, like FAFSA, Sallie Mae, etc. then you don't need to pay the deposit. Didn't you mention that you're using Federal Loans? I would've preferred to use FAFSA myself but my course falls under Medicine and the only form of financial aid I could use with that was Sallie Mae.