The Available Functions window includes a list of functions that can be used when creating an expression.

Filter The Filter option determines which functions are shown in the list of functions. The All Functions filter shows the entire list of all functions whereas the other filters show only those functions that can be used with a particular field type. For example, if the expression being defined is evaluating a date field, then it will be useful to filter out the functions that cannot be used with date fields and show only the date functions.
List The list of functions shown depends on the Filter specified.

Note: When using functions, each function begins with a function name, followed by parentheses. Values within the parentheses are used as conditions for that function. Some functions support multiple conditions, separated by commas. Functions can be "chained" by using other functions as conditions. All functions return either numeric or string values; string functions have a dollar sign ($) appended to their names.
 
For example, the expression DATE$(DATENUM( ) + 7) uses two functions, DATE$( ) and DATENUM( ), and the addition Operator. The result of this expression is a string representation of the date seven days after the current system date.

The Insert button will close the window and insert the highlighted function into the expression being defined. Double-clicking a function in the list does the same thing.

The Cancel button will close the window without changing the expression being defined.