A page title for users can be different from a directory name with files of page. So file systems have a restriction on possible characters for files and directories. For example you can't use in Windows follow characters: *, ?, :, /, \ and some others. But this characters can be in page title with an alias. OutWiker shows page alias in the notes tree if page alias set.

When user creates a page with forbidden characters or duplicate page title of the same level an alias will be set automatically. A page folder will be corrected to satisfy file system requirements. But user will see entered title.

An alias set in a page settings file (__page.opt) in section [General] with an "alias" parameter.

In a main OutWiker user interface is no way to directly change this parameter. Usually users do not need this. The plugin HackPage allows to add and change page alias.

You can use alias (or directory names) to create links to other page, but it is recommended that you create links using identifiers.