In the *Object* Settings menu you can find and enable the Cross Object feature.

The Cross Object configuration can be found in the Cross Object Tab in *Object* Settings.

By default, Duplicate Check maps the most logical fields between Account, Contact, and Lead. If you want to map additional fields click on the 'Add Cross Object' button. 

Choose which fields you want to compare between the two different objects. You can map any field to any field.

Watch this short video of how to enable and map fields in the Cross Object feature.

Duplicate Check can only add scenarios fields. For example, when using Lead to Contact, DC will always use the scenario of the second defined object (in this example: DC will take into account the Contact Object scenario).