Mix is a third party product of Requisis, a German company.  We met Requisis at IOT Exchange and ended up purchasing a few years ago when migration options were very limited. Our decision predates many of the updated migration features in DOORS 9.   Mix is expensive.  It has simplified our work and we've been happy with  support, and the product once they released 1.2. Mix installs  DXL in DOORS 9, and uses OSLC to sync with ERM/DNG. It is GUI based and pretty good at automapping, and allows manually mapping which helps with RDF URL problems.  It remembers mapping (via it own DOORS 9 modules) and links so updates are possible.    I haven't tried other products nor kept up with the D9 migration improvements so can't compare Mix to other options.  My only relationship with Requisis is as a Mix customer.

 As far as I can tell, Requisis is an independent 3rd party vendor.