A trigger is a set of actions that is executed once an issue meets certain conditions. For example, a value for one of the issue's parameters exceeds a certain threshold or a certain user subscribes to the issue.
A trigger can modify an issue's parameters, change its status, leave an automatic comment or send an HTTP request.
Each queue has its own set of triggers. Triggers are executed in the same order they were specified in queue settings. However, you can manually change the trigger order if necessary.