Examples of such IDs are Publisher Provided IDs (PPIDs), DMP specific IDs or other types of internal ID types that you want to use when building query audiences for audience sync & real time targeting via the Opable SDKs.
How to use custom identifiers.
There are ten custom identifiers that you can point specific ID types to, they are represented by the IDs c to c9 which you will reference as headers when loading data into your DCN.
You can also assign each custom ID a "friendly name" which you can use to identify which custom ID is for which purpose. For example, if you have a PPID that you want to load into your DCN, you can use the custom ID c with the friendly name My PPID
Managing your Custom IDs
To load this PPID into your DCN, you would then associate your PPID to the custom ID c by formatting your file or Snowflake table based on the tabular data schema with the header id_c in column one.