The following documentation describes about Event Variable Discriminator.
Working with Events and Alarms - CA Spectrum - 10.2 - CA Technologies Documentation
You can blank the field in the Event Pair dialog if you don't have Event Variable Discriminator. In my own words, Event Variable Discriminator is Event Variable that contains unique Trap data to make alarms distinctive. E.g. suppose you have a trap that has been mapped in Spectrum and it raises Printer Fault alarm. And there is variable binding (trap variable data) in that trap that contains Printer Fault Type information, such as toner low, empty toner, paper jam, etc. Say the trap variable binding is mapped to Event Variable 10 in AlertMap then you want to set that Event Variable as Event Variable Discriminator. Why? If you don't set the Event Variable Discriminator then when you have 2 different traps, e.g. 1st trap is paper jam trap and the 2nd is empty toner trap, then you will only have one alarm in spectrum with the 'more' blue link (it indicates the same alarm happened multiple times). If you set the Event Variable Discriminator then you will have 2 different alarms because now Spectrum distinguishes the alarms based on value of Event Variable 10 (Event Variable Discriminator).
If you set Event Variable Discriminator, please make sure you configure the event to clear the alarm also with the same Event Variable Discriminator.