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When clicking a subnavigation item all four (better five) frames are reloaded. In each frame the script is called that is configured (via record in table frame_files) to be displayed in the given frame for the given area.
Location
For each mainnavigation item a location is stored as an XPath expression describing where to find its caption in the navigation.xml file.
navigation.xml
This file can be found in ROOT/contenido/xml/ and contains a structure of CONTENIDOS main- and subnavigation and calls to the i18n function which allows for translating the navigation item captions.