SQCS 7 requires CPA firms to define the criteria for an engagement quality control review. I discussed that here.
The criteria have to be objective. You look at an engagement, compared to the criteria, then decide if you need to do a review.
The criteria cannot be written such that it is purely up to the discretion of the engagement partner. The criteria cannot be that partner feels like it, has time, does or does not have budget left, or is just uncomfortable on this engagement. The criteria need to be objective.