PSA is a server application and not a plug-in, which allows you to sign PDF documents without any manual intervention. This is useful for signing of large number of PDF documents in an automated way. This server application can interface with a PKCS#11 HSM, for cryptographic processing and does not require Adobe Acrobat® to be installed. You can configure settings like hand signature, background logo, page where signing to be done, signature appearance location, reason, location and much more.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.