The fine folks at Google have found people struggling with site management when it comes to DFA. To combat the confusion, they have created a handy tutorial that should help out.
On the Google Ads Developer Blog, the team lays out the two types of sites developers will deal with in DFA:
Site Directory sites are accessible to everyone who can view the Site Directory. These entries contain information about the site that all DFA users share, such as whether the site accepts mobile and/or interstitial placements. You cannot use these sites directly when creating placements.
DFA Sites are your own personal pointer to a Site Directory site. They contain settings for interacting with this site that are specific to your account. These are the sites with which you can associate placements.
Developers will have to map a new DFA site when they begin making placements for the first time. There are two simple steps to follow when setting this up. The first step is "Locate the site directory entry for this site." You can do this by using the getSiteByCriteria operation, but this is to only be used when mapping additional sites to your account. You should use getDfaSites for everything else.
The second step is creating the new DFA site. After getting the site ID from the first step, you can make the DFA site. After this, you can modify personal settings for the newly created site.
Hit up the blog entry for the code you need to Google+Ads+Developer+Blog%29">implement these features. Happy coding!