Where to find the reference? The Canadian AIM publication, IFR charts and FAA Advisory Circular 120-42B, and FAA Advisory Circular 91.70A.
The FAA designates Canada's NCA and Arctic Control Area (ACA) as AMUs. Although Canadian publications sometimes refer to it as the area of compass unreliability, they are the same. The magnetic North Pole is at approximately 75°N 100°W and is slowly moving as it circles the true pole every 960 years. This is why we see current navigation charts occasionally changing an instrument landing system (ILS) course by 1°.
When operating in the AMU, move the HDG REF switch to TRUE when the Canada HI 4 chart defines the course with a °T. Add two additional items to the master flight plan checklist: TRUE HDG adjacent to the first true heading leg and MAG HDG at the end of the AMU. This will serve as a reminder to return to a normal heading reference. The primary reason for selecting TRUE HDG in the NCA and ACA is to provide a more realistic navigation display (ND) heading presentation, thus avoiding rapidly changing heading indications. This will help with radar vectors in TRUE and comply with Canadian Air Regulations.
Note the comments in parenthesis on the chart below:
The worst magnetic compass performance is probably in the center of the Northern Control Area, home to the magnetic North Pole. This position creates a notch in the circle of magnetic unreliability, often called a "key hole." Charts should be checked for the presence of a "T" denoting the use of True Heading instead of Magnetic. Some aircraft automatically switch to True based on airway designation or latitude. The G450, for example, automatically switches above N73° or S60° latitude. More about this: G450 Procedures & Techniques / FMS: True/Magnetic Selection.