You may have noticed when programming an NA series HMI, that the behavior field of a toolbox item varies between items. Below shows the properties of a momentary button and data lamp. Where the momentary button uses a Variable for its behavior, and the data lamp uses an Expression for its behavior. 

What is the difference between the two?

The title of this field varies from Variable, Expression, and Value, depending on the object or item.

• A Variable field is mainly used by objects such as Data Edit, or Button that write values to the HMI. You can only set a variable in this field.

• An Expression field is mainly used by objects such as Data Display or Lamps that read values, specify startup conditions, or display targets. You can set arithmetic expressions in addition to variables.

• A Value field is mainly used by the objects that write fixed values or variable values. While you can just enter a variable in the Variable field, you can also enter an arithmetic expression. Thus, you can enter a value obtained by adding a fixed value to a variable field.