My sub-specialty was (retired) neonatology, the care for sick newborn infants. I guess, since most of my research endeavors dealt with lung problems, I was sub-sub-specialized.
My orthopedics group that my wife and I go to has folks who concentrate on spine, shoulder/elbow, knee/hip/ankle, etc. Someone whose area of expertise is this narrow range usually has a greater breadth of knowledge within that area. Does not necessarily mean that he/she is better than a more generalized orthopedist, just more focused...
Hope that helps.