How do I use resource 204?

In one of the last updates it said that by using resource 204 a unit could always be visible, but I don’t understand how to implement this effect in age3 so that it always shows an enemy unit by putting the resource 204 and the unit ID but in negative.
but the enemy unit is not shown.

What am I doing wrong?

Another question: Can this resource also be used to display units in the Scenario Editor?