Item (ITM) content is the term used by Autodesk to describe the content components that are native to Autodesk Fabrication CADmep software, Autodesk Fabrication ESTmep software, and Autodesk Fabrication CAMduct software products. An item (or ITM) file is a single piece of content that represents the geometrical dimensions and the associated data of the manufacturers’ product.
ITM content is designed to hold the graphical and non-graphical information needed for modeling, fabrication, and installation of ductwork, pipework, and electrical containment systems. ITM content is intended to replicate the manufacturers’ information as closely as possible, allowing for real-world, constructible modelling.
ITM patterns are based on the needs of fitting construction parts, such as sheet metal fittings, as well as procured parts, such as pipework and electrical containment. The pattern (referred to as a ‘CID’ inside the Fabrication products) can support dimensional calculations, material assignment, part numbers, and connector construction, while connectivity can be driven by predefined specifications or pressure classes.
ITM content can now be natively used inside of Autodesk Revit 2016 software and Autodesk Revit MEP 2016 software products (referred to as "fabrication parts" in the Revit user interface). ITM content databases can be developed inside of any Fabrication software product and then used inside of the Autodesk Revit software products to help detailing mechanical, electrical, and plumbing systems to LOD 400 specification.
Once you understand how ITM content works, you may want to review the information on Creating Content.