EventCodes is a member of the APLTree library. The library is a collection of classes etc. that aim to support the Dyalog APL programmer. Search GitHub for "apltree" and you will find solutions to many every-day problems Dyalog APL programmers might have to solve.
Dyalog uses numbers to represent errors. Those errors are accessible via ⎕EN. However, when one wants to specify an error with a :Trap or a ⎕TRAP statement then one must use integers.
Although a seasoned programmer will over time memorize the most important error numbers, the unusual ones will escape her, not to mention newbies.
There is a better solution to this: using symbolic names which are effectively constants (strictly speaking read-only variables) carrying the integer. That's what the EventCode class is for: it offers symbolic names for all trappable events up to and including version 17.0.
Examples:
EventCodes.DEADLOCK
1008
EventCodes.GetName 1008
DEADLOCK
EventCodes.List'T'
TIMEOUT 1006
TRANSLATION_ERROR 92
TRAP_ERROR 84
⍴EventCodes.List''
53 2