The first part of an event template defines the event type of suitable events (in this case Tick), while the section in brackets describes filtering criteria that must be applied to the contents of events of the desired type for them to match.

In the example at the beginning of the chapter, the first parameter within the event template has been set to a wildcard (*), specifying that all Tick events, regardless of the value of their name parameter, are suitable. That is, as long as their second parameter, price, is greater than 10.The filtering criteria supplied are applied to the event's contents in the same order as within the event definition for that event type. This is known as positional syntax.

Specifying parameter constraints in event templates lists all the filtering operators (like >) that can be applied to the value of a parameter within an event template.