External requirements are those Requirement elements that have been connected to the current element using a Realization connector. By creating the connector from the element to the Requirement, you create an expectation that the element must implement the requirement as part of the system solution.
In Enterprise Architect, linked Requirement elements are shown in the 'Links' tab of the 'Properties' dialog for the element.
Right-click an external requirement in the list and select the 'Locate Related Element' option to highlight the Requirement element in the Project Browser. Press and click on that element to show its properties in the Properties window.