Indicates that the style extensions in this resource are essential to the interpretation of the styled text and that it cannot be rendered without the style without losing essential information.

If set to true, all systems that claim to support this extension and which render elements of the resource containing this extension SHALL either render the content as required by the style extensions defined by this profile that are present in the resource or shall (if safe to do so based on their own rules), omit entirely the styled content they cannot render in accordance with the extensions.

unique id for the element within a resource (for internal references).

An Extension

Source of the definition for the extension code - a logical name or a URL.

The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition should be version specific. This will ideally be the URI for the Resource Profile defining the extension, with the code for the extension after a #.

Value of extension - may be a resource or one of a constrained set of the data types (see Extensibility in the spec for list).