An expression combines field names, constants, and operators in a calculation that returns a single value. You can use an expression in a variety of commands to assign a value to a temporary field or Dialogue Manager amper variable, or use it in screening. You can combine simpler ones to build increasingly complex expressions.
When you write an expression, you can specify the operation yourself, or you can use one of the many supplied functions that perform specific calculations or data manipulation. These functions operate on one or more arguments, and return a single value as a result. To use a function, you simply call it. For details about functions, see the Using Functions manual.