you just use the value "Unit - Custom value of (Insert Unit)" which returns an integer value. The value (known as index or id) refers to the unit . This way, it is easier to create spells so array variables may use this id to refer the unit.
i've tried this with custom and normal game models of trees, it need more model info. so it makes it white or a green and black block that can't turn into a normal tree after you save and open the map again.
I don't know what his global dummy means but, I think from what I've done that global dummy is a unit that casts/have spells that are used for the entire map and specific units or all units. Because I use it.