This attribute is used to set the View that will provide options when linking records using Relationship fields. If a ViewId is not used the table indicated in the TableId attribute of the <View> element will be used for linking options.
<View Id="Heroes" TableId="Contacts">
<Relationship Id="Enemies" TableId="Contacts" ViewId="Villains" />
<View Id="Villains" TableId="Contacts">
<Relationship Id="Enemies" TableId="Contacts" ViewId="Heroes" Reciprocal="true" />
In the example above, Contacts are defined in "Heroes" and "Villains" Views. Each Views defines a Relationship field of enemies. In the "Heroes" View - the ViewId="Villains" is used so the selection list when choosing Enemies will display records from the Villains View.
For Villain records, ViewId="Heroes" is used so Enemies field will display options from the Heroes View.
If ViewId="Villains" were to be removed from the <Relationship> tag in the Heroes View - the options for choosing Enemies would include all records in the Contacts Table (from the attribute: ><View Id="Heroes" TableId="Contacts"
This list would include Heroes, Villains, and possibly other Contact Types such as Volunteers, Authors, and Baby Seals.