What i understood from various post on forum is that CIC has prepared profiles based on their risk assessment (countries visited ,associate orgs ,names etc) based on VO's assessment they can ask for inputs from any single or multiple agencies -CSIS or CBSA or other source ..and that could be the reason why some apps sails smoothly while others goes in tailspin due to multi-agency clearance ...
IRCC refers Security Screening request to CBSA. CBSA forwards an SS request to CSIS. But CSIS report is only part of overall SS. IF CSIS reports was everything, there are several applicants who, including Matt, whose CSIS is completed for months, should have gotten the PPR by now.
Most Canadian Missions abroad have a liaison office (which is either manned by CBSA, or CSIS or RCMP e.t.c) within it.. For example in Pakistan this office is manned by RCMP (surprised?) (RCMP Liaison office in Pakistan is also responsible for Afghanistan, Iran, Kazakhstan, Tajikistan, Kyrgyz, Turkmenistan, and Uzbekistan)
http://www.canadainternational.gc.ca/pakistan/defence-defense/index.aspx?lang=eng
One purpose of this office among many other functions is "
facilitation of information exchanges between Canadian law enforcement and Pakistan law enforcement with regards to all aspects of criminality ranging from terrorism and organized crime to drugs
and immigration". So, for me, since i am a Pakistani (and for any Afghan, Tajik, Iranian, Krygz, Turkemani or Uzbek), CBSA, in addition to a request of ss check by CSIS on me, will, also forward a request to RCMP liaison office in Pakistan to run a background check/local verifications on my application.
Once CBSA receives the report from CSIS and RCMP (some other agency can also be involved on request of CBSA) office in Pakistan, it will prepare an admissibility report on me and forward it to IRCC, which have the final say. Please note that in many other missions, instead of RCMP, CBSA or CSIS runs such liaison offices, and in those cases, CBSA will forward request to whichever agency from CBSA or CSIS is running that liaison office for background verifications that needs to be conducted in applicants home country.