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.