Publishing an i-model
An i-model is a V8-format DGN file generated from an original design file. A published file contains both graphical and business/component information from the original. Some graphical information is converted to a simpler, more efficient representation in the published file. The business/component information is converted into a common form that does not require access to the original WorkSpaces or WorkSets, or connections to external project databases.
If a design file has references, publishing it creates multiple .i.dgn files: one for the master file and one for each of its displayed references.
In the published file, the file extension .i.dgn is appended to the design file’s entire file name. For example, if you publish a file named Elements.dwg, the published file’s name will be Elements.dwg.i.dgn.
You can also publish files which have references to protected DGNs. You can publish protected DGNs if you have the right credentials (password and/or certificate) and the DGN grants the rights to "export." If the publishing process encounters a reference to an i-model, it ignores the i-model and does not republish it. However, when creating a package of the published file, the referenced i-model is included in the packaged file.