Trust groups will probably want to have their own marking definitions beyond TLP or Statement. DHS is working on one for AIS, which Marlon has asked me to write up using SEP.
I’m not sure the currently defined concept of SEP exactly fits with introducing a new marking definition type. If you look in the spec - for the definition property of a marking-definition it says , which of course is poorly defined. The statement and tlp definitions aren’t SDOs or SCOs. Adding a new marking definition object isn't the same as adding an extension, custom object or custom property.