This page details an example of how Publica can be used to pass contextual data.
In the Publica Ad Server publishers should use the available custom parameters to pass in the "iris_context" and "iris_id". Currently, Publica has custom parameters that range from CUSTOM_5 to CUSTOM_8.
For example, if you have CUSTOM_5 and CUSTOM_6 available, you'll use CUSTOM_5 for the "iris_context" and CUSTOM_6 for the "iris_id".
Here's how the two parameters would be formatted once passed: