Some types of event support the FLTRBLK parameter on the ENFREQ macro to identify the address of an optional Enf Listener Filter block. The filter block data is specific to the signal type and identify further criteria to be used to determine if the signal should be delivered to the Enf listener. ENF Event code 51 supports filter blocks that allow further filtering of GRS contention data signals to ones that involve specific and patterned qnames and rnames that might be of interest to that Enf Listener. The Enf 51 Listener uses the FltrBlk keyword to identify the address of the filter block for what signals are to be filtered further by the supporting Enf 51 signaller beyond that of signals matching just the Qual and QMask keywords.