@qbteachmt figured out before I did that it's a reference to the standard per diem rates, that are issued by the federal government (GSA) but used by private enterprise and accepted by IRS.  Travel nurses work temporary assignments wherever hospitals have shortages.  Sometimes for only a week, more often a month or two at a time.  I would ask the client, to confirm that this is "information only" and means that the amounts are not included in Box 1 wages.  

If the "traveler" has no tax home, should this be reported as compensation?  Let your conscience be your guide.