In my experience, The is an important difference.
An air release valve will be one set up to separate gas from liquid, typically using a float system. These do not necessarily open at any set pressure or pressure differential, only by the buoyancy of the float.
An air relief valve will typically be set up as a valve set to open at a particular pressure, and designed to deal with the flow of gas (rather than primarily liquid).
Relief valves are for pressure, release valves are for phase separation. Note that neither of these is a precision device for control. If you need good pressure control or really good phase separation, there are better ways to do it.