NR-73 is the form available to individuals to determine the status of residency when leaving Canada. NR-74 is the form available to individuals to determine the status of residency when arriving to Canada.
It is not necessary to fill out the forms, unless the individual were asked by the Revenue Canada Agency. The purpose of completing an NR-73 is if an individual wants opinion on residency status, which is determined case by case (i.e., for each individual based on their individual circumstances).
If an individual were asked to fill out an NR-73 or NR-74, the individual should see a professional tax consultant in order to do it properly. Do not let the Canada Revenue Agency to decide for you to determine your residency status as the decision might not be at your favour. The advantage of completing an NR-73 is that the individual gets an opinion from expert about his/her residency status for tax purposes, and can rely on that opinion in determining whether to continue to file Canadian tax returns or not. This is a complex area of Canadian tax law and we recommend that if an individual were asked to complete the NR-73, the individual should consult with a tax professional with emigration expertise.