The main purpose of Canada's NSOR is to assist a police officer who is investigating, or to help prevent, a crime of a sexual nature.
The criteria for this group was that they must have still been serving an active portion of their sentence on the date that the SOIR Act came into force (i.e. still incarcerated) on probation, or on parole.
These retrospective offenders were tracked down by various law enforcement authorities, and were served with a Notice that they were required to register for the NSOR after a one-year grace period.
Since April 15th, 2011, individuals who return to Canada and who have been convicted of a sexual offence outside of the country, must report this fact within 7 days to the nearest police force.
Depending on the nature of the offence, they may receive notice that their name will be added to the NSOR, and that they will be required to comply with the same reporting obligations under the SOIR Act.