Use Undersize Bearing Usage Guide: Notes

WARNING: This page is about a different car, the 2006 Nissan Titan. However, it is still accessible from the selected car via links, so may be relevant.

Use undersize (US) bearing when the oil clearance with standard size main bearing is not within specification.
When using undersize (US) bearing, measure the inside diameter of the bearing installed and grind the journal until the oil clearance falls within specification.