Makes a field optional.
If a field contains a value in both data sets, it is used as usual.
If a value is present in only one data set, it is ignored and not counted as a break.
For example, consider the following two records:
Let's assume that Data set 2 does not always contain the "Rate" field. With the optional rule attached to this field, Duco will use this field when it is populated for both data sets and ignore it otherwise.
*Do not use the Optional rule in cases where absence of information may be due to errors. Such errors will not be detected if the rule is present. The rule should be reserved for scenarios where matching on a field genuinely can be optional.