The type of element for which this element type can substitute for.
Enables the element to appear anywhere the substituteForTypeName element can appear in a UXML document.
For example, if an element restricts its children to Button elements (using the uxmlChildElementsDescription property), elements that have substitueForTypeName return Button are accepted as children of that element.
The value of this property is used for the element's substitutionGroup attribute in UXML schema definition.