Okay, but the section of the Access to Information Act might help to understand what sort of information is being withheld. Here is what the law says:
https://laws-lois.justice.gc.ca/eng/acts/a-1/page-3.html
Starting from the bottom,
meeting minimum requirements "may" simply mean that you're not a Canadian citizen, permanent resident or subject to any of the limitations like having an ongoing refugee claim or being outside of the one year prohibition for applying.
Criminality, security and medicals are really issues after receiving AIP, so applicants who have been in Canada for a lengthy period of time, after a refugee claim has been unsuccessful have likely had some sort of screening that aligns with the claim. There could be adverse information there, but it would have likely surfaced inside the claim. It would still be redacted, but the applicant would probably have had to address whatever it may have been. It may not be adverse to be redacted.
CSO support.., this came after my time but Google explains it as:
The Operations Support Centre (OSC) processes:
- International Experience Canada work permits,
- In-Canada applications submitted online,
- Verification of Status (VOS) applications,
- Replacement of valid temporary resident documents,
- Amendment of immigration documents and of valid temporary resident documents.
This appears to me to be very low level decision-makers who organize and distribute applications to their proper destinations. Maybe they digitally screen applications and only enter information? Replacing existing TR documents suggest a very limited authority, so it looks to me to be a simple screening and distribution operation.
Missing pages.., this could be a few things, including an error on the part of the human who has the most tedious work within the system - reading everything to see what can and cannot be released.
It's worth reading the legislation to see what they'll limit in a release, but common (to me and my very dated experience) in the immigration world could be information about specific third parties, and they could be unknown to the applicant.
An example could be that an employee at the CBSA, IRCC or Refugee Board wants to know something from your home country. The Canadian diplomat (CBSA, RCMP have diplomatic positions abroad, for 2 examples) responsible for that nation may have been engaged with any sort of question. That could likely be redacted if it's considered an "ongoing" investigation. The investigation could be considered "ongoing" but is not really being pursued. It might be general information produced internally and not releasable because it identifies Canada's communication(s) with who knows who. This sort of "general report" might be attached to all applicants from a specific country (I don't know this, but it wouldn't surprise me if the RPD did something like that).
It's guesswork - work that never ends. I would add that there are two types of information or evidence. Intrinsic and extrinsic. If information that is not readily available to the applicant but IS in the hands of the decision-maker, then this extrinsic information has to be disclosed under the principle of natural justice if it's a consideration within any refusal.
Sometimes someone from work, someone jealous of your relationship or a neighbour who doesn't like someone will write a poison pen letter. That would be redacted.
You just don't know what you don't know, but in the many, many, many immigration case files that I've looked at, it was rarely heart-stopping. I don't think that you were formerly a corrupt union leader, an agent of the SAVAK or an embezzling head of a foreign bank. If you were, you wouldn't ask what it is and instead you'd ask, who's it from?