Every wiki can have a specific group of people who can access it. This lets you set up a wiki structure that fits your organization's security requirements.
For example, if you work in an organization with top-secret projects, you'd want each of those top-secret projects to have its own wiki. If you have departments where people in a department work on a few of those secret projects but not all of them, you'd give them access to the department wiki and specific project wikis.
When you add content to a wiki, make sure the content is appropriate for the people who can access the wiki.
For example, if you added secret project information to a department wiki, everyone in the department could see the information, whereas people outside the department, who work on the secret project, don't have access to the department wiki, and so they wouldn't have access to the secret project content. When you have content that doesn't fit current wikis, create a wiki for the content.
When using wikis, be aware of which wiki you're currently viewing. When you search or use tags while viewing a wiki, you'll find pages within that wiki but not in other wikis. However, if you're viewing My Page, you can search or use tags for pages in all wikis you have access to.