The Prefix property contains a string that acts as a filter when the contents of the Dictionary object are saved (usually to a database). When the name of a name/value pair begins with the specified prefix, that name/value pair is not saved. By default, the prefix is "_", so that any keywords beginning with an underscore character are excluded.
The pipeline component that saves receipts usually sets the prefix to "_cc_", so that all credit-card information is not saved in the database. (The actual string used by the pipeline component is configurable.)