Slicer  5.3
Slicer is a multi-platform, free and open source software package for visualization and medical image computing

Widgets → MRML Relation

File in Modules/Loadable/Markups/WidgetsIncludes file in Modules/Loadable/Markups/MRML
qMRMLMarkupsAbstractOptionsWidget.hvtkMRMLMarkupsNode.h