Background checks usually start when you recv your medical request. This process does not involve the applicant but Canadian and other agencies, where Canada Checks that the applicant is no threat to the Integrity of canada and is not involve in any kind of activities that threats the security of canada i.e Terrorism, espionage, membership of a banned group, military service, govt official etc.
Standard BC takes 30-90 Days after Medicals
In Depth BC takes 30-365 days
When You recv your AOR from CHC that means your documents have been assessed and you are ok to go.