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 listed in the Responsibilities window and on the 'Requirements' tab of the source element's 'Properties' dialog, and from there they can be located by right-clicking on the name and selecting either the 'Find in Project Browser' option or 'Find in Diagrams' option.
External Requirements are also shown in the 'Links' tab of the 'Properties' dialog for the element, and again can be located in the Browser window by right-clicking on the external Requirement in the list and selecting the 'Locate Related Element' option.
When an external Requirement is highlighted in the Browser window or in a diagram, its properties are displayed in the Properties window (press to display this window if it is not open).