What is a PDF signature field?
Definition
A signature field is an interactive PDF form field reserved for a signature: a specific region of the page that captures or holds a signer's mark, separate from ordinary text fields. It marks where a person is expected to sign.
Signature field vs. typed text
A typed name in a text box is just characters. A signature field is purpose-built for signing: it designates the spot, can be required before a form is considered complete, and keeps the signature distinct from the form's other data.
Why it matters on a form
Detecting the signature area is part of making a form fillable correctly: a signature line should become a signature field, not a generic text box, so the finished document treats the signature as what it is.