Looking at the parameters of various families you might notice that some seem to be missing their Omni Class parameters. Taking a closer look you should note that these are probably system families like walls, floors, ceilings, roofs, structure foundation slabs etc. The text from the Revit help documentation:
OmniClass is a new classification system for the construction industry. The Autodesk Seek website 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 have parameters for assigning an OmniClass code, except for the System and Annotation families. [emphasis added]
You can access the OmniClass Number and OmniClass Title parameters in the Family Category and Parameters dialog under Family Parameters. See Family Category and Parameters.
The description on the home page of OmniClass is:
The OmniClass Construction Classification System (known as OmniClass™ or OCCS) is a classification system for the construction industry. OmniClass is useful for many applications, from organizing library materials, product literature, and project information, to providing a classification structure for electronic databases. It incorporates other extant systems currently in use as the basis of many of its Tables – MasterFormat™ for work results, UniFormat for elements, and EPIC (Electronic Product Information Cooperation) for structuring products.