OmniClass is a new classification system for the construction industry. The web site uses codes from OmniClass Table 23 to filter and identify shared content. A code consists of an OmniClass number and title.
If an OmniClass code is not already assigned to a family, you are prompted to assign one during the sharing process. However, you can continue to share with Autodesk Seek without defining one. All Revit families (except the System and Annotation families) have parameters for assigning an OmniClass code.
Taken from here.