Reflection probe importance.
The bigger the value the more important the probe is. When deciding which probe to use for the object, Unity will always pick Important probe over the Not Important. This property also affects blending, if object is inside both Important and Not Important probe, only Important probe will be used, when object is leaving Important probe bounding box, it will gradually blend over Not Important probe. If both probes would have same importance, the object would use both probes.