The Supreme Court in KSR Int'l Co. v. Teleflex Inc. (2007) explained that noted that “because inventions in most, if not all, instances rely upon building blocks long since uncovered, and claimed discoveries almost of necessity will be combinations of what, in some sense, is already known,” “it can be important to identify a reason that would have prompted a person of ordinary skill in the relevant field to combine the elements in the way the claimed new invention does.” The required “expansive and flexible approach” may look at a variety of facts, including prior-art teachings and marketplace demands and artisans’ background knowledge, “in order to determine whether there was an apparent reason to combine the known elements in the fashion claimed by the patent at issue.”

However, such an analysis supporting a rejection under 35 U.S.C. 103 obviousness "should be made explicit." 

In the instant case, the Federal Circuit noted that the PTAB merely agreed with Apple's contention that "a person of ordinary skill in the art reading Woodhill and Stefik would have understood that the combiantion ... would have allowed for the selective access features of Stefik to be used with Woodhill's content-dependent identifiers feature." 

But that reasoning seems to say no more than that a skilled artisan, once presented with the two references, would have understood that they could be combined, said the court. 

And that is not enough: it does not imply a motivation to pick out those two references and combine them to arrive at the claimed invention. See Belden Inc. v. Berk-Tek LLC, 805 F.3d 1064, 1073 (Fed. Cir. 2015) (“[O]bviousness concerns whether a skilled artisan not only could have made but would have been motivated to make the combinations or modifications of prior art to arrive at the claimed invention.”); InTouch Techs., Inc. v. VGO Communications, Inc., 751 F.3d 1327, 1352 (Fed. Cir. 2014).

For the full case and opinion -- http://www.cafc.uscourts.gov/sites/default/files/opinions-orders/16-1174.Opinion.2-10-2017.1.PDF