Server roles in SQL Server 2016
These terms are not new to SQL 2016, but they are also not server roles as far as I know.

For all i know these are pre-built Parallel Data Warehouse resource classes.

There isn't much documentation on that except some blog posts.

Have a look at these


PDW, Integration Services and Resource Classes
A teched presentation from Channel 9


I only heard about these in some presentation though, I have never worked with PDW so I can't help you any more than this.

Edit

Just adding in one additional resource (direct link to download PPT)
SQL Server 2012 Parallel Data Warehouse. You are basically controlling who gets more "speed" with processing by what role they are members of, at least from what I gather. The last slide in the deck:



Edit

In the meantime an article on workload management has been posted in the Azure documentation: Concurrency and workload management in SQL Data Warehouse where the following is mentioned (thanks Bob Klimes):


  SQL Data Warehouse exposes four different resource classes in the form
  of database roles as part of its workload management implementation.
  
  The roles are:

smallrc
mediumrc
largerc
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