object trigger placeholder format?

Trying to find productive areas to play with this - not a veteran with triggers.  Has anyone figured out the placeholder format for IT Asset fields inside object triggers?

Is it documented anywhere?

The trigger editor prefills this format when you hit View available placeholders:

{{custom_objects.standard::itam_asset.custom_fields.<new>}}

I've tried a number of guesses to replace “<new>” and I get nothing in my email bodies.

Real pointers or better guesses appreciated!