If you are on H4 Visa in US and applied for extension or change of status using the new Form I-539 form with Biometrics process, you would have received H4 Biometrics Appointment notice from USCIS. H4 holder would need to go to the Application Support Center (ASC) and give the biometrics on the given day and time. When the H4 holder goes to the ASC, they need to fill a form and go through the process of giving Biometrics. After the process is complete, as proof of completion of Biometrics, the ASC stamps the Biometrics Appointment letter. It looks like below. You need to verify the Date, Location and Time to make sure you have the right details on it.
Sample H4 Biometrics Completion Stamp given by ASC as Proof
You can see the stamp in the bottom part of the H4 Biometrics Appointment Notice. It has the date of the Biometrics, location of ASC info, signed by the ASC official.