The External References action shows an overview of all the (external) references, which have been found during the AIM scanning. This includes references to (local) files and external websites.
As said, the reference itself (the url and the link type) is registered during the AIM scanning. However, in a separate background process, the LinkSpider verifies these references.
The last checked date column indicates the last time the LinkSpider has verified an external reference and the link state column holds te result. Link state Found indicates that the reference could be found and is valid, any other state indicates the reference could not be verified succesfully, which usually means there's something wrong with the reference.
You can use the filters on the right to zoom in on specific types of external references and/or reference with a specific link state.
Select a reference to view more details, such as the number of times it's in use. Click the Details button to get a more detailed overview, including a list of items in which the reference is in use.
When it's a (local) file reference which happens to be an image as well, the image is included within the details. The items listed within the In use by list can be clicked, which will open a contentitem infobox with details about the item.
The details for an external reference which couldn't be found/resolved usually include additional information about the problem:
Click the Rescan button if you want the LinkSpider to rescan every external reference. You will be asked for confirmation, before all the external references are going to be rescanned.
When you do confirm, all external references will be reset (last checked and link state info will be set to null, retrycount is set to 0 and the link state is set to Undetermined) and the LinkSpider is signalled to start its process right away.
Previous versions of the External Reference action did have a Remove Orphans button, to manually remove orphaned external references (references which are no longer in use). This button has been removed, the LinkSpider will always remove orphaned references as part of its process.