I wondered this. Here is the result of my research.
From Cisco document:
http://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k_r4-0/system_management/configuration/guide/b_yc40asr9k/b_yc40asr9k_chapter_010.pdf
"Secure domain routers (SD...