Ad Sitelinks, is search-based ads feature that allows you to extend the value of existing AdWords ads by providing additional links to specific, relevant content deep within your website. Rather than sending all users to same landing page, Sitelinks can provide up to 4 additional links for user to choose from. By providing users with more options, you can create richer, more relevant ads that improve value of your brand terms and other targeted keywords. [More: AdWords Help Center].
"Sitelinks is supported with AdWords API v201003. Sitelinks is implemented as campaign ad extensions, and can be managed using CampaignAdExtensionService. Keep in mind that each campaign can have only one CampaignAdExtension containing a SiteLinkExtension. This means prior to adding Sitelinks to a campaign, you should check if campaign already has one, and remove it if does. C# code snippet below shows how to retrieve campaign ad extension containing active Sitelinks for a given campaign," explains Google.
Full Article embedded below:
Discover v201003: Using Ad Sitelinks