Motion buffer range is a range that prevent unit targeted abilities and attacks from canceling when the target moves out of cast or attack range during cast time, or before reaching the attack point. For abilities and attacks, the default motion buffer range is 250. Few abilities have different values.
Abilities with different range
Items with buffer range
Although items do not have cast time or have 0 cast time, a few items have 250 buffer range to make them castable within default cast range + buffer range. But if the target is out of this range, it will be cast until the target moves within default range.