apias:

... I think next year Pirelli need to bring an Ultra Soft tire.

I was just joking about this at the time, but...

... 

The latest plan is for Pirelli to bring a default prime tyre, for example the soft last weekend in Sochi, and teams will be able to pick two other tyres from the range.

So you might select three sets of super soft plus three sets of the new “ultra soft”, about which not much is known as yet. It will give teams enough tyres to have some strategy options. ...

-- http://www.jamesallenonf1.com/2015/10/analysis-pirellis-new-formula-1-deal-more-variety-more-testing-better-racing/