Medium where the certificate is provided on
Adobe demands PDF signatures to be delivered on secured hardware that protects the private key. This hardware has to apply to strict requirements, to prevent the certificate from falling in the wrong hands.
USB-token
A USB-Token is used for signing periodically.
Hardware Security Module (HSM)
When certificates are being used for automated signing, an HSM is required. HSM's are available as a PCI card or as server implementation, and have to be purchased apart from the signature.
USB