This function is used to test if a custom field is defined by specifying a name.
The supplied name may contain any of the escape sequences described in Escape Sequences.
The function is only executed once per set of files while executing stepwise.
The action test state is set to true if the custom field is defined, otherwise it is set to false. A Custom Field is a Credit named variable is set as follows: If the custom field is marked as being a credit, the named variable will have a value of 1 otherwise it will be empty.