smp:Certificate

Hi,
I have some questions on smp:Certificate:
/ProcessList/../Endpoint/Certificate  is
defined as  Holds the complete  [X509v3]  signing
certificate of the recipient gateway, as a PEM base 64 encoded DER
formatted value.
1)  Where is this certificate used?   I could imagine it would
enable a sender to validate that a signed receipt is using the
correct certificate.  But since SMP already requires an out-of-band
trust model for sender certificates,  I'm not sure why the option
would be there for receiver certificates.
2)  Since the definition is explicitly about signing certificate,
how does a sender know which encryption certificate to use for a
particular receiver?
Kind Regards,
Pim