The only question is whether "registering" citizenship means "applying for", which appears to be the same thing as the steps, timeline, and requirements are identical.
I believe that "registering" refers to people who are Canadian citizens born abroad and have to register with immigration to get their status recorded. "Retaining" refers to the old rules of 2nd generation born abroad who would lose their citizenship by the age of 28 unless they applied to retain it.
If you look at the consular manual for citizenship:
4. Roles and responsibilities
For clients outside Canada, consular missions provide general information and case specific information on citizenship certificate (proof) and renunciation applications. Consular missions accept citizenship certificate and renunciation applications on behalf of CIC, forward completed applications to the Case Processing Centre in Sydney, Nova Scotia (CPC Sydney), do follow-ups with clients where required and provide country specific expertise, especially with regard to document authenticity.
Questions on adoption applications and statelessness grant applications should be referred to visa offices.
Adult grant applicants are expected to reside in Canada and no service is provided to these applicants outside Canada.
Subsection 20(1) of the Regulations authorizes consular and visa officers to administer the oath of citizenship to persons outside Canada who have been granted citizenship under sections A5(2) (minor grant), A5(4) (discretionary grant) and A11(1) (resumption).