Event B1 is being used for Inter-RAT measurement report triggering event.
As per specification: Event B1 is used when Inter-RAT neighbor becomes better than the threshold. There are following inequality conditions are given which need to be fulfilled.
Entering Condition: Mn + Ofn - Hys > Thresh
Leaving Condition: Mn + Ofn + Hys < Thresh.
where "Mn" is measurement result of Inter-RAT neighbor cell and "Ofn" is frequency specific offset of the frequency of the inter-RAT neighbor cell. "Hys" is the hysteresis parameter for this event (i.e. Event B1)