Both the Navigation Pane and the TOC display heading styles and other styles with outline levels by default.

To have a style appear in the Navigation Pane and not in the TOC you need to have separate styles. Any styles to appear in the Navigation Pane must have an outline level as a part of their paragraph formatting. That is the only criteria for the Navigation Pane and it is not optional.
Using the Navigation Pane in Word 2010 and Later

The TOC, on the other hand, has numerous options. By default, it, too, uses outline levels but you can, instead, or in addition, specify certain styles or even use TC fields.

• TOC Tips and Tricks by Suzanne Barnhill, MVP
• Generating a Table of Contents - Complex Documents
• How to create a table of contents in Microsoft Word by Shauna Kelly