When you design a form that is connected to a database, the resulting data
source (data source: The collection of fields and groups that define and
store the data for an InfoPath form. Controls in the form are bound to the
fields and groups in the data source.) consists of query fields and data
fields.
Query fields should be used to create controls that allow users to enter
values that they can query the database with.
Data fields are filled in with the query results, and also allow users to
edit existing records and add new ones.