Alerting Services

Flight plans serve multiple purposes. One of the purposes is to allow ATC to anticipate my arrival in their airspace, making it easier for them to provide traffic separation and other services. In Canada, except for VFR arrivals at a busy airport, that function is pretty much exclusively for IFR flights. For both IFR and VFR flights, flight plans make it clear where search and rescue should start looking for me if I don't show up at destination. Those two functions are filed together in one flight plan but can be perceived to be separate when an IFR flight plan is cancelled. Cancelling IFR to continue a flight VFR doesn't turn your flight plan into a VFR plan, as you might expect. Instead you have cancelled your flight plan, leaving only alerting services remaining. The difference between a VFR flight plan and alerting services, I discovered, is that you can't amend alerting services to reflect a revised destination. If you need the flexibility of a VFR flight plan following IFR work, you have to file a Y flight plan that actually has separate filed IFR and VFR portions.

The other way to get alerting services without activating a flight plan is to take off without a clearance. Unless From the AIM:

7.10 Alerting Service IFR Departures from Uncontrolled Airports

At locations where communication with ATS is difficult, pilots may elect to depart VFR and obtain their IFR clearance once airborne. In Canada, if IFR clearance is not received prior to departure, SAR alerting service is activated based on the ETD filed in the flight plan. However, if departing from a Canadian airport that underlies airspace delegated to FAA control, then responsibility for SAR alerting service is transferred to the FAA and FAA procedures apply. In such cases, alerting service is not activated until the aircraft contacts ATS for IFR clearance. Therefore, if the aircraft departs before obtaining its IFR clearance, alerting service is not provided until contact is established with ATS.

Once upon a time a Canadian flight plan, and thus alerting service, was not opened until the pilot opened it, but to close the window of vulnerability from a departure accident, flight services will now "assume you off" at the filed departure time. If your departure was significantly late or early, you can call in and amend their recorded departure time. That's important because the time they judge you overdue is based on adding your estimated time en route to your departure time.