It’s been pretty defined. Chung is the starting strong safety and plays primarily in the box, but they will drop him in Cover-2 occasionally. He’s had a fairly solid year in coverage, but struggles against big tight ends. Still, he’s been much better in his second stint in New England.
On passing downs they’ll pull Chung for Harmon who has better range on the back end. Sometimes they’ll even play all three of them.
McCourty has done just about everything this year. Usually, he’s the free safety playing Cover-1, but he will come up and man cover tight ends sometimes. He’ll even occasionally blitz.
Tavon Wilson is behind Chung at the strong safety spot, but has picked up some spot duty throughout the season. Not as much as his rookie year in 2012, but definitely more that 2013.