PM -602-0157: Contracts and Itineraries Requirements for H-1B Petitions Involving Third-Party [open pdf - 119KB]
"This policy memorandum (PM) establishes U.S. Citizenship and Immigration Services (USCIS) policy relating to H-1B petitions filed for workers who will be employed at one or more third-party worksites."
U.S. Citizenship and Immigration Services: http://www.uscis.gov/