What does the /:x: and /r mean in SharePoint Online sharing URLs?

Ever wondered what the new SharePoint Online URLs are all about?  Take for example
What exactly does the /:x: and the /r between the domain and the /sites mean?  Well wonder no more...

/:b: PDF
/:f: Folder
/:i: Image
/:o: OneNote
/:p: PowerPoint
/:t: Text
/:u: Undefined
/:v: Video
/:w: Word
/:x: Excel

Happy coding...