Unit Is Created can also be used for when you specify the action - Create Units in your trigger. This means any action - Create Units that you have created in separate triggers or actions, you can specify that unit using the Trigger event - Unit Is Created, by adding a condition - Created Unit == The Unit that is created through the action - Create Units.
Say for example,
Event - Map Initialization
Action - Create Units With Default Facing - Hero A at Point...
Event - Unit Is Created
Condition - Created Unit == Hero A
Action - Here, you specify the actions for Hero A. Eg. Execute Effect A to Target Unit (Hero A) from Caster Unit (Hero A).
If, for example, your unit is created through data and not through Triggers, e.g. through a train ability.
Then, you will need to create a dummy effect which you can use via triggers.
Put the dummy effect under the Unit : Effect (Birth).
Next, create a trigger with Effect Used Event specifying the dummy effect.
Create a global variable which serves to mark the Unit.
Under the Effect Used Event Trigger, set the global variable == Triggering Effect Unit (Caster).
This means that during the Unit : Effect (Birth) event when the unit is created through the train ability, the following actions runs and so on.
Lastly, another alternative is to use Data Tables where you can save a Unit into a string and later use it in your trigger.