On the extensibility page in the Modifier Extensions section, will add: "When defining a new modifier extension, it is designated as being a 'modifier' rather than a 'regular' extension by setting the "isModifier" element on the root 'element' for the StructureDefinition to 'true'.  Only extensions that are defined as being modifiers can appear in modifierExtension.   A regular extension cannot be 'constrained' to be a modifier." On the StructureDefinition page, will add a new section called "Root elements", with the following text: "The very first element in a snapshot (the one with an element id of just the type name) defines characteristics that apply to the type as a whole.  Some of these characteristics affect the constraints that can hold on an element that references the type.  For example, if the root element cardinality is "0..1", then an element declared to be of that type cannot have a maximum cardinality greater than 1.  (This is often used when defining extensions to indicate that multiple extension repetitions of that type are not allowed within a single element.) In some cases, there can be redundancy between content defined on the root element and that defined on the StructureDefinition itself.  For example, ElementDefinition has 'definition' while StructureDefinition has 'description'.  These may have the same content put both places, though technically the 'definition' is text that describes what the type 'means', while 'description' is broader and may talk a bit about what the resource/data type/profile is for, in addition to its meaning."