You can open an IGES file in Autodesk Inventor (creating a file), import an IGES file into an existing Autodesk Inventor part file, or place an IGES solid body as a component in an assembly.
When you open an IGES file, 308 and 408 structures are used. The block structures in the file are translated into multiple parts referenced by an assembly. Each part file contains the data for a single structure instance.
When you use 
Open and select an IGES file, click Options to set import options. On the Import Options dialog box, specify the data to import and other conditions to be applied.
Choose the data to import by selecting Solids, Surface, Wires, Points, or any combination.
If the Import into Repair Environment option is selected, the imported data is placed in the browser as a repair body composite.
If the Composite Feature Mapping option is selected, the imported data is placed in composites.
If the Construction Group Mapping option is selected, the imported data is placed in a Construction folder that contains individual group, solid, surface, and wireframe nodes. Surfaces are contained in a single node to help manage them for promotion in Part modeling. You can promote multiple surfaces as a single feature.
Composite features have the same behavior as Base Surface features, but they can also have multiple individual solid and surface bodies within them. While the composite can contain multiple bodies, you cannot work directly with the individual bodies. You can only interact with the composite as a whole.
If possible, multiple surfaces are stitched together into a single part. If edges do not match exactly, the surfaces are stitched into the smallest number of quilts possible.
Surfaces that cannot be stitched together remain in the Construction folder in the browser. Quilts are promoted to the part environment and are shown as surfaces in the browser. In the construction environment, you can analyze and repair surfaces that did not automatically stitch when they were imported.
When opening an IGES file that contains 308/408 subfigure (block) definitions, an assembly file is created using the 308/408 (block) figures to define the assembly structure. If the Auto Stitch and Promote option fails or is turned off, the data is placed in construction groups in the defined part.
Optionally, you can ignore the 308/408 subfigure (block) definitions and open all the data into a single part file. To do this, open a new part and click the Insert menu, and then click the Import option to import all the data into the new part doc.