This would be a big help when editing a feature class with the Collector App. The idea being that the selection of a certain subtype in one domain would trigger the selection of a subtype in another domain based on the selected value. For example if you have a domain listing types of cover crops in on domain and the user selects a value of "None" then the value of another related domain could be set to "N/A". This could improve the speed and accuracy with which features are collected in the field.