I'm assuming you mean for spouses/common-law applicants?
I don't know the details and there's no exact science to it, but basically, they have to check that:
- the sponsor qualifies to sponsor the applicant (mostly done in Mississauga, but the visa office also checks this, especially in the case when the sponsor is living abroad at the time of the application)
- the applicant qualifies as a member of the family class (check that the marriage is legal, the common-law union exists, the sponsor had not excluded this person from being sponsored - like for example if the sponsor landed in Canada without declaring a spouse back home, ...)
- the relationship between the sponsor and applicant is genuine
- there are no security reasons why the applicant should not be admitted into Canada (this includes previous convictions)
- the applicant passed the medical exam and is not a threat to public health
I might've missed something, but I think that's the gist of it. And it goes roughly in that order.