As many of you have pointed out, CPIC is a complicated animal. Once you are charged, you receive a FPS # that is stored in CPIC for good. There are 4 levels to it and nobody knows for sure what the US has access too.
We do know that once you have a criminal record, it is stored in CPIC for good on level 1 unless you get a pardon. A pardon will seal the record but it will still show up as a sealed record on level 4. A lot of people get charged and later get discharged, acquitted or have charges dropped, this info is complicated to remove. A file destruction may help but solely done at the discretion of the arresting police department. There is also people who have had several police interactions but never charged. Some of this info can remain on CPIC for a lifetime until they pass away or actually never. For example, people who had a suicide attempt may be flagged as a person of special interest on CPIC. A pardon sometimes does not remove the FPS # and everytime you cross the border, a CBP agent can deny you entry. The federal government recently made changes where CBP agents cannot have access to some parts of the special interest category for people who had interactions for mental illness.
Level 4 would be for the vulnerable sector check. Every charge even if dropped or acquitted and any police interaction such as a suicide attempt will be revealed on level 4. This has caused a great hardship to people who are now putting pressure on provincial and federal governments to make changes often with objections from police forces. When it comes to job applications, level 1 which is basic record check will not reveal any record if a pardon is granted. However, any job that requires working with vulnerable people will reveal any police contact on level 4. What police agencies reveal on level 4 is at their discretion. A sealed record needs approval from the federal government to be revealed. Police agencies will not reveal a sealed record if it is not related to the job itself for the vulnerable sector check because the federal government denies their request. People with dropped charges, discharges, police contact, peace bonds or once investigated but never charged do not need a pardon and all the info is revealed by police agencies on the vulnerable sector check. Changes are being implemented to stop this practice by police agencies.
As many have pointed out, CBP can see you were once fingerprinted and deny you entry while looking through the CPIC levels. A higher ranked CBP agent can access info on that system that a regular agent cannot and take someone into secondary. CBP officers will then use several techniques into making a Canadian traveller confess about their past. There are also many other databases.
1) APIS, a system where everyone travelling over the US even though they never intend to land will be required to submit all their info onto that database. Travellers who fly over US airspace will be entered on APIS 24 hours prior by every airliner. A criminal record will be stored in APIS for good as travellers are being checked by Homeland security.
2) There are also other databases that are shared between local canadian police forces and US authorities which store someone's info for good. Several provinces have agreements with US states that share information for traffic violations and so much more.
3) Interpol's database will store for good all info of foreign travellers who have been involved in serious criminal activities. All countries have access to Interpol. A canadian pardon or file destruction never destroys such information. Fraud convictions are stored on Interpol as well as serious drug convictions and sexual crimes. This is why many people could be denied entry into Mexico.
4) Canada may have granted access to basic level 1 check of CPIC to some countries but we are unaware of it. Off course, they will never tell us. You have the UK, Australia, New Zealand, Canada and US who are part of the 5 eyes meaning they have access to info about each other's citizens.
You have more chances of never being denied if you take the necessary steps.
Once convicted never travel until you get a pardon. Then try to get a file destruction to remove your FPS # if not destroyed. Now If you were involved in serious criminal activity you may be on Interpol so there's not much you can do. If your charges were dropped, you were acquired or discharged make sure you remove as much info as you can within CPIC through a file destruction before you ever travel.
The whole system is mostly made to punish people who were never convicted. Convicted people can use other resources to gain access to the US or move on.