They're apparently trying to use him the way they used Everson Griffen who is now a fulltime DE. Griffen was a pass rushing specialist who'd line up at DE or even inside at DT on passing downs. Barr is playing LB but them moves to the DL on passing downs. Right now primarily DE but they're tinkering with him at DT too, again only in obvious passing downs.
They're expecting him to be the starting OLB opposite Greenway when the season starts and then rotate into the DL. Zimmer has been very positive on Barr said he's raw but he's going to be absolutely special when he figures everything out. They've said there have been days in camp when they literally can't block Barr on passing downs, run defense is a work in progress but he can get after the QB.