This property is actually inherited from ICollection<T> interface which is defined in the .Net Framework. So this refers to the ability of adding members to your collection. Now regarding Studio File Type Framework a segment can be a collection of other items and act as a container for other elements. This is done by inheriting ISegment interface from IAbstractMarkupDataContainer.  Now the default implementation of ISegment is throwing the exception you see in the debugger but for example an ITagPair should return a true/false value. The reason why the segment implementation is throwing an exception is because although a segment is a markup container it really acts more like a holder of markup items or other markup containers.

This property is actually inherited from ICollection<T> interface which is defined in the .Net Framework. So this refers to the ability of adding members to your collection. Now regarding Studio File Type Framework a segment can be a collection of other items and act as a container for other elements. This is done by inheriting ISegment interface from IAbstractMarkupDataContainer.  Now the default implementation of ISegment is throwing the exception you see in the debugger but for example an ITagPair should return a true/false value. The reason why the segment implementation is throwing an exception is because although a segment is a markup container it really acts more like a holder of markup items or other markup containers.