What is the difference between One-way FLEX and Return FLEX?
One-way FLEX
One-way FLEX vehicles, also known as the “regular” FLEX option, allow trips to start at point A and end at point B anywhere within the FLEX service area of the city or region where the car is based. With these vehicles, it’s possible to begin a trip at one location and finish it at a different location within a large territory.
Return FLEX
Return FLEX vehicles are assigned to a smaller zone, typically an area of a few city blocks that can be quickly covered on foot. The zone for a particular Return FLEX vehicle is indicated on the map in the Communauto app. Return FLEX vehicles must be brought back within the boundaries of their zone at the end of the trip. Otherwise, they enjoy the same parking privileges during the trip as One-way FLEX vehicles.
At the moment, Return FLEX cars are only available in Montreal.

The information above is also available in the icon legend found in the Communauto app (bottom left, just above the Explore button).
See also:
Is there a difference in price for One-way FLEX and Return FLEX trips?