If you fly a unit somewhere, treat the unit as if it had Airborne while crossing borders, e.g. .
Most cards use fly (or a synonym like "throw through the air," e.g. ), as forced movement. In these cases, e.g. a unit being moved by Avatar of Air's ability, it will be able to cross the border of a or , and will not take damage from a , even if it's disabled, silenced, or is in the realm. Note that this effect is specific to crossing borders (and not entering sites), and therefore will still trigger for non-Airborne minions moved via Avatar of Air's ability.
Alternately, sometimes a unit flies itself, e.g. or , in which case it is not forced movement. See the forced movement entry for details.
If you fly a site somewhere, e.g. , treat all units occupying the site as if they had Airborne while the site moves.