.  A number to the right. This is a zero based index of the layer (the first layer is number 0, not 0). If you need to enter a layer number in the event system, this is the corresponding number. (You can also enter layer names in the event system which might be more convenient if you regularly reorder layers.)  Some quick experimentation reveals that to refer to a layer by name, you would use quotation marks, while a variable or other expression with the same name can be referenced without quotation marks.  A number to the right. This is a zero based index of the layer (the first layer is number 0, not 0). If you need to enter a layer number in the event system, this is the corresponding number. (You can also enter layer names in the event system which might be more convenient if you regularly reorder layers.)  Some quick experimentation reveals that to refer to a layer by name, you would use quotation marks, while a variable or other expression with the same name can be referenced without quotation marks.  