Custom targeting is a useful tool to enable targeting of specific types of pages without having to build a ridiculous amount of ad units. It is necessary for having great targeting capability while having a lean DFP setup.
Related Read: DFP Cheat Sheet for Beginners: Targeting Line Items
Below are the directions for setting up custom targeting:
Step 1: Log into DFP
Step 2: Click the Inventory tab
Step 3: Click Custom Targeting
Step 4: Click New Key and name it — this is the title of segmentation variable (eg. category, demographic)
Step 5: List each variable (one per line)
Each variable should be placed on the relevant page. When you target a custom variable when creating a line item, it will show up on the relevant pages that have that variable. For reference, here is a publisher that has custom targeting set up: view-source:http://geebo.com/jobs-online/list/.