@gloney
The server you gave, a1094-135786059.cluster247.canvas-user-content.com, is matched by the first item on the resource page you linked to.
Under Canvas Core Domains > Browser Security, the item is *.canvas-user-content.com.
The * in front is a wildcard. It allows anything as long as they end in .canvas-user-content.com. It does not require that there be a single hostname in front of the suffix, it can be a hostname with a subdomain as in this case. As long as it ends with the .canvas-user-content.com then it is okay.
As the name suggests, the content on this server has been uploaded by a user.