Though I don't see any restriction on where this can be used, it aims for building Salesforce apps.
> What is the Lightning Design System? It is collection of design patterns, components, and guidelines for creating unified UI in the Salesforce ecosystem.
It was also mentioned at the time Microsoft's Office UI Fabric project got released. It is very likely that both projects aim to help third party developers get a consistent look, when developing add-ons for their applications.
> What is the Lightning Design System? It is collection of design patterns, components, and guidelines for creating unified UI in the Salesforce ecosystem.
http://www.lightningdesignsystem.com/faq