Most expressions are made from functions, such as the Copy function in the above example. Here are the types of Expressions:
Functions - Created when a function is dragged to the expression tree or to an output map element.
Map Element Reference - Created when a map element is an argument of a function expression. There can be references to either input or output map elements.
Function Argument - Represents a formal parameter (argument) to a function. This returns the result of its child expression, which is the value of the argument.
Functions can have a fixed or variable number of arguments. If the function has a variable number of arguments, there will be no function argument expressions when you drop the function on the expression tree. A variable argument function expression can have any number of child expressions.
A fixed argument function results in a function expression having a function argument expression for each of its arguments. Each of these arguments takes exactly one child expression, and if the child expression is not specified, the map cannot be executed (you will get an error explaining the problem).