If you have a 7.1 receiver and want to configure it for 6.1 usually you use the Surround Rear Left terminal and that is usually specified in the manual.
You tell the receiver you are configured for 6.1 the same way you tell it 2.0, 3.0, 3.1, 5.1, etc - by setting the speakers to Yes or On (whatever terminology the receiver uses).
You'd set Front Left, Center, Front Right, Surround Left, Surround Right, and Surround Rear Left to On/Yes and you're all set.