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You're developing a website that uses the Persistent URL (PURL) module, possibly in conjunction with the Spaces or Sites modules. PURL creates separate namespaces in your Drupal site, and through modules like Spaces, your website has some nodes/views/pages in some spaces and some content in other spaces.
However, the URL Alias module requires paths to be unique across the entire site. This is a requirement at the schema level, and is enforced in the UI. This means that you can't natively have an "about-us" path point to node/47 in one space, but point to a different node (or view/page/etc.) in another space.
PURL Pathalias attempts to solve this problem by replicating the Path module's functionality, but with PURL namespace awareness. Thus, you can have something like "name1.example.com/about-us" point to node/47, but "name2.example.com/about-us" point to node/48.