$TriggerObject usage according to documentation

[Anonymous]

I'm researching an implementation of an email script that accesses custom objects and noticed the "$TriggerObject" in this documentation​​​. Although, the documentation seems to suggest that I should see the Trigger Object fields in the UI because it says:

When using the $TriggerObject, fields must be checked in the editing pane for the object in order to be made available to the script.

The email script edit pane requires you to check the fields that you want to use, but there is no reference to a trigger object? I've triggered an email through the "Added to {Custom Object Name}" trigger (the documentation mentions this) and attempted to access fields on the "$TriggerObject" (like the documentation), but it appears the TriggerObject is not defined?

My hope is that I could access the "Edge" object in this type of setup​, but I can't figure out what "$TriggerObject" is and if it really exists. I tried the following where "groupClassRegistration" is my "Bridge" object and it's field is checked in the script editor for debugging purposes.

ID1: ${TriggerObject.marketoGUID}
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ID2: ${TriggerObject.MarketoGUID}
<br/>
Else: ${groupClassRegistrationV2_cList.get(0).marketoGUID}

[Anonymous]

I have the same set up of Lead -> Product Interest -> Products, and email scripting won't show any Product fields in the editor. Given that it's a many-to-one relationship between Product Interest and Products, you'd think you'd be able to start at the intended Product Interest record and traverse to the Product record to pull the name, price, description etc into your email.