A Notice of Reassessment (“NoR”) is very different from a Notice of Assessment (“NoA”).
When you file your tax return, the CRA will process your return and issue you a NoA, usually within two to eight weeks. The NoA typically confirms the amount of tax that you owe based on what you reported in your tax return.

But if the CRA later conducts an audit and concludes that they do not agree with your initial tax return (perhaps they believe you have unreported income or they deny your business expenses), they will issue you a NoR. The CRA can issue you a NoR years after they issued your NoA and they often add significant tax and penalties.

You have the right to object to the NoR (and taxpayers often succeed), but you have to file your objection before the time deadline. For more information, read here.