This is one component of an AnalyticalModelSketch, which exists to provide greater granularity over the Analytical Model than at the Element level.
Assembly: RevitAPI (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)
This may or may not be a simple one-to-one mapping to ElementIds. For instance, one Element may be defined by multiple Curve Elements within a Sketch. But another Element may not have that designation, and the individual curves are not defined by any other Elements within a document.