timestamp – What are the main constraints x9.95 imposes on RFC3161

So x9.95 seems to be a trusted timestamping scheme built on top of RFC3161.
I couldnā€˜t find a good definition though how it differes or what additional constraints it imposes.

Can someone list the (technical) constraints that a x9.95 system fulfills which are not generally fulfilled by a system that adheres to RFC3161?