WAU (n)~ An upgrade option that allows users to upgrade from one edition of Windows to another by purchasing the upgrade online. It is enabled by Software Licensing Services (SLS) technology which allows a digital lincense to determine what features are enabled or disabled allowing Microsoft to sell either additional licenses or product upgrades through digital licenses (PUP-DL). Customers may either buy full packaged product (FPP) or upgrade quickly and conveniently electronically.