I don't think CIO is considered as primary office. But there are some cases which are entirely processed in CIO then for those candidates CIO Sydney or Ottawa acts as primary office.
But this is what I have learnt, generally all applications go to CIO Sydney or Ottawa and after R10 checks they are sent to different VOs which are normally the primary office to do the background checks. If the application needs further review then they are again sent to local VO(LVO) of the candidate, this is secondary office. "Ready For Visa" email can come from either one offices.
In Sept thread I read one applicant got 3 "Ready For Visa" emails. He had 1 primary office and 2 secondary office and all 3 sent that email lol
So in my case, my primary office is CPC-Ottawa. I don't have a secondary office yet as per my GCMS notes but if I get one in future then it will probably be NDVO.