SIGNATURE adds a digital signature: this is a security feature that EscapeE can add to make PDF documents tamper-proof, and enables signatures to be validated. See Security options for PDF export.
Digital signatures need the certificate name and a serial number; they may also include a description. E.g.
<SIGNATURE CERTIFICATE="DGF LLC" SERIAL="7C 56 05 3F 88 42 13 88 45 95 4A 2B 03 C7 56 47 " DESCRIPTION="Test file">
[digital signature encoded in base 64]
IDF helps with setting these up: check the "Sign" box when creating the IDF file.