UMLR — No rules on Extension Pts governing differences between Use Case definitions & «extend» relationships usage

Extension points appear to be documented in the properties of a use case. However, extension points are also allowed to be referred to in the condition note attached to the «extend» relationship. Is it legal to refer to an extension point that is not defined in the use case? Does this automatically add the extension point to the base use case. An example of this is in figures 18.3 and 18.11