This code system http://hl7.org/fhir/extension-context defines the following codes:

CodeDisplayDefinition
resourceResourceThe context is all elements matching a particular resource element path.
datatypeDatatypeThe context is all nodes matching a particular data type element path (root or repeating element) or all elements referencing a particular primitive data type (expressed as the datatype name).
extensionExtensionThe context is a particular extension from a particular profile, a uri that identifies the extension definition.