Per CBP, an IIT is defined as:

Section 322(a), Tariff Act of 1930, as amended (19 U.S.C. §1322(a)), provides that "[v]ehicles and other instruments of international traffic, of any class specified by the Secretary of the Treasury, shall be granted the customary exceptions from the application of the customs laws to the extent and subject to such terms and conditions as may be prescribed in regulations or instructions of the Secretary of the Treasury." 

To qualify as an IIT within the meaning of section 322(a), Tariff Act of 1930, as amended (19 U.S.C. §1322(a)), and the implementing regulations, section 10.41a of the CBP regulations (19 CFR §10.41a et seq.), an article must be substantial, suitable for and capable of repeated use, and used in significant numbers in international traffic (e.g. reusable containers).  Paragraph (a)(1) of section 10.41a designates IIT lift vans, cargo vans, shipping tanks, and certain other named articles and states that other articles may be designated as IIT by the Commissioner of Customs in decisions to be published in the weekly Customs Bulletin and are available in the Customs Rulings Online Search System (CROSS) at http://rulings.cbp.gov/. 

Articles qualifying for treatment as IIT under 19 CFR §10.41a are considered to be used in international traffic when they arrive in the U.S. containing merchandise and when they arrive empty to be filled with merchandise to be exported.  If qualified, upon arrival into the customs territory of the U.S., the IIT may be released without entry of the payment of duty subject to the provisions of 19 CFR §10.41a.  To attain the release without entry as specified above, the applicant requesting release must have a valid 3a bond on file (19 CFR §10.41a(c) and 19 CFR §113.66).  The presence of a quantity of residue remaining in the conveyance upon its return to the U.S. does not disqualify the article as an IIT.