Answer
When creating a PDF form, it is best practice to generate premade signature field areas for all users to sign to retain document security and so all users can sign easily in order. If a user does not have an area to sign, they can still use the certificate signature option to draw the signature area on the PDF.
When filling out a PDF form, you also have the option to lock down and apply document restrictions and protections to the form when the last signature field is signed:
If there are still signature fields available on the PDF file, this option is not available or is visible to the signer: