You can create the appearance of a virtual site (or "sub-wiki") by configuring a set of spaces to share the same Zen components.
To create a virtual site, in each space you want to include, use the Zen Space Layout screens to set the same space to provide the master page layouts, menu, header, and footer. Also, set the Click Target For Logo to the same space (the space providing the master pages, etc. is usually a good choice).
Create virtual sites for for departments, divisions, project teams, and even external customers or partners who need access to certain portions of your Zen wiki.
Branding Space Clusters
While not required, you may want to create special versions of your Zen brand for each virtual site. This will visually reinforce your virtual site separation, keep users oriented, and can be used to strengthen brand relationships with external customers and partners.
What About Space Categories?
Confluence has a "Space Categories" feature. Space Categories are a way of conceptually grouping spaces so they appear within a single tab in the Space Directory (great for site administrators). Space categories can also be used to adjust scope for content searches.
For more information on Space Categories, visit Atlassian's Confluence documentation.
Example
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