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 -> https://jenyay.net/Outwiker/HackPageEn]] allows to add and change page alias.

You can use alias (or directory names) to create [[links -> page://wiki_links]] to other page, but it is recommended that you create links using identifiers.