Event Template is a parameter defined by Genius Vision to remedy the fact that some parts of ONVIF are vague or not clear enough about actual implementation of event notification mechanism. This, in reality, causes the developers to imagine and implement ONVIF in several slightly different ways and thus won't interoperate altogether.
As one of our principals stated, and because no specs are perfect that are without flaws or contraditions, we created this additional parameter in order to workaround such spec issues and provide several alternative implementation to accomodate these compatibility issues.
The summarized definitions of each Event Template can be looked-up here.
Following is an example of how to configure Event Template for a particular NVR channel.