We propose to add an explanation along the following lines:   In FHIR, extensions are represented as elements in pre-existing arrays designated for this purpose. Every resource has an extension and a modifierExtension array at the top level, inherited from DomainResource. Every element in the resource also has a pre-existing extension array, inherited from [Element] ( https://www.hl7.org/fhir/element.html ). These arrays contain elements of the [data type Extension] ( https://www.hl7.org/fhir/extensibility.html ). In FSH, extensions are created using [extension rules] (#extension-rules). These rules have the effect of adding the extension to the corresponding FHIR extension array, technically, by slicing that array. The extension is referred as a member of the extension array, using its name or URL in square brackets.