After you file
I-797 notices: receipt vs. approval
I-797, "Notice of Action", is how USCIS communicates by mail. After you file, you first get a Receipt Notice (I-797C) with a 13-character receipt number (3 letters + 10 digits) confirming your case is pending — keep this safe, it is how you track the case. Later you may get an Approval Notice (I-797 or I-797A). For an extension, the approval "valid until" date becomes your new status expiration and replaces the old I-94 Admit Until date.
What if…
I have a pending I-539 from before — does that matter?
It can. Having another I-539 already pending while you prepare a new filing is a situation a licensed immigration attorney should review before you file. Tell the assistant about it and upload the I-797C receipt so your case reflects it.
Official sources
This is general information, not legal advice.