Nonstandard Container Rules

For FedEx Ground shipments 70 pounds and under if I select the Nonstandard Container option a $10.50 fee is added to the Estimated Rate. For shipments over 70 pounds the Nonstandard Container option does not change the rate. I am not using any dimensions in the rate. What are the exact requirement (weight or dimension) where the Nonstandard charge applies? I know it's only valid for Ground shipments

Thanks.