Google recently released a new version of its DoubleClick for Publishers API. One of the more interesting features of the new version was the addition of custom fields. This new feature allows users to include "additional information on DFP entities without having to implement data persistence in your own system." Since it's a new feature, some people might not be getting the hang of it.
To help ease people into using custom fields, Google posted an explanation with a quick tutorial to help get people into using the feature. It explains the data types that can be used alongside an easy walk-through that shows an example in Java.
Custom fields only support one of four data types. These data types are string, number, toggle (true/false) and drop-down. Google feels that these four data types should cover the most common use cases that developers will be running into while using custom fields.
To help create and manage your new custom fields, you can enable the new CustomFieldService on your network. The service must be turned on before you can start creating custom fields. Custom fields are also coming to the user interface at some point in the future so it should be getting easier from here on out.
To help get you started, they also posted a lengthy example of string and drop-down values being added to a custom field. The example is in Java, but custom fields are also available in Python, PHP, .NET and Ruby. You can view respective examples at the Google's library example page.
Here's the example Java code for custom fields:
Even with all the extra help, I'm sure that custom fields will still be a point of confusion for a lot of DFP API developers. Unfortunately, the last DFP API Office Hours was held on May 9, but the Hangout should be going up on YouTube within the next few days. There won't be another for a while so hopefully somebody else asked your question during the last Hangout.
If you want to brave it alone, that's fine too. Here's the documentation for custom fields to get started.