Records in EDS  - the records that comprise the result set of any given search - come from the enabled databases in any given EDS profile. These records are either just metadata, or they are metadata plus content in the case of records with an attached HTML text document.  The records - the metadata - are in XML format.  Below is a partial example:

Open

To view a record in its XML format, insert 'xmlonly=Y&' into the URL that appears in the browser address bar while you are viewing the detailed record of a result, between 'detail?' and 'vid=' :

Open

Links in EDS construct themselves from this record metadata, mostly the metadata contained within the <controlInfo></controlInfo> tags.  Links are composed of two parts - a base URL and a query string, and they search the 'Local Collections for Filter' selected in the maintenance screen for that link (which also has the fields for base URL and query string). Custom link maintenance screens are accessed at (Linking tab → Custom Links/Modify → Setup/Maintain Custom Links → choose link to modify). 

Base URLs are sometimes static, but often they are dynamically generated from the metadata.  Query strings are always dynamic. Query strings use either DOI or OpenURL syntax, or a conditional mix of both.  Records in EDS rarely contain permalinks, PURLs, or other direct links because both EBSCO and publishers want the flexibility to alter their domains or other elements of their servers that would render such static links inoperable.  Once the link is constructed, it is sent to either an EBSCO server if the resource is held in a proprietary EBSCO database (such as Complementary Index or Academic Search Ultimate), or to a publisher platform/database.

Which links appear for any given record is determined by the information in the <externalLinkInfo> tag, specifically the <linkName> child tag(s). These <linkName> tag items are matched to the Custom Link array in EBSCO Admin for that profile (example below from Linking tab → Custom Links/Modify):

If one of the <linkName> tags correspond to one of the custom links arrayed for that profile in EBSCO Admin, it will select the first link that matches as it descends the list in numerical order.  Then that link will attempt to execute according to its base URL, query string, and local collections specifications.