There are two types of links: standard and user-defined.
Standard links are split into the following categories:
By default, all links except labels, are drawn as icons in the Scene View. Labels are drawn as text.
User-defined links enable you to customize the link categories to suit your workflow. By default, user-defined links are drawn as icons in the Scene View.
If available, user-defined links are drawn as icons in the Scene View by default.
You can use the Options Editor to toggle the display of each of the link categories, and also to control their appearance.
When you add a link, you can assign a user-defined category, hyperlink category, or label category to it. The rest of the categories are automatically assigned by Autodesk Navisworks when it generates corresponding links.