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You should review the notice of assessment (NofA). Although filing a notice of objection is always an option there may be other remedies.
The notice of assessment has an explanation section describing changes to the return. The Canada Revenue Agency (CRA) may need more documentation to validate a claim or disallowed a claim in error. After reviewing the NofA you can contact CRA to discuss their assessment. Some issues can be resolved by filing an adjustment or supplying the documentation CRA needs to substantiate a claim. Generally CRA is more than happy to adjust issues where a valid claim is rejected.
A notice of Objection can be filed if the above steps fail. CRA and a taxpayer may disagree about an interpretation of the tax code. In that case a notice of objection can be filed to have the claim reviewed.
Hope this helps