Don't make the width too condensed
Make sure the width is generous enough so that it doesn't feel too condensed. This ensures it doesn't get confused with buttons or other controls.
Use descriptive titles
Navigation should be helpful. Choose titles for navigation items that clearly communicate the places where they'll go. Arbitrary or non-useful titles cause usability issues.
Be concise
Along with being descriptive, the labels of navigation items should be succinct. Keep navigation strings to 1 or 2 and no more than 3 concise words (in U.S. English, which is the source locale). Reduce any unnecessary words in order to ensure simplicity.
Navigation items should never be so long that they require truncation, except in instances where navigation is user-generated, such as folder and filenames.
Be cautious with line breaks
When possible, the default width should automatically adjust to the longest string in the navigation to accommodate all translations. As a last resort for long strings, specific line breaks can be built into the implementation. These breaks depend on the content and require manual handling by globalization engineers.
Mixed icon usage
In multi-level side navigation, icon and text-only styles can be used together. Users may include supporting icons on the first level and omit them on the second and third levels, or choose to use no icons at all.
Do not mix icon and no-icon options within the same navigation level.
Avoid deeply nested menus
The multi-level side navigation goes to three levels deep. Adding more than three levels will make the indentation indiscernible, which becomes a major usability issue.
Use consistent multi-level behavior
If top-level navigation items have a location associated with them, send the user to that location and open the sub-level navigation items. If a top-level navigation item does not have any associated location, only open the sub-level navigation items.
Side navigation can use either of these behaviors, but should never mix behaviors in the same experience.