Another tip, or FAQ, from our support engineering team:
Since the release of adCenter API v6, we?ve gotten some questions about the use of CustomerID in getting and setting campaign level targeting.
?Why is CustomerID mandatory for this API and not for the others??
In the version 6 release, targeting was enabled at the campaign-level, and using the object model, the target setting can be refined at the ad group level. Shamit Patel, our program manager for API, explained this well in the blog post on Waterfall Targeting http://community.microsoftadvertising.com/blogs/api/archive/2009/10/13/adcenter-api-campaign-level-targeting-quot-waterfall-targeting-quot.aspx
One of the advantages when coding is that you can create a single target object, save it in your customer library, and use it across all accounts. This is first feature for which we?ve enabled this model. It should be more efficient, however the CustomerID is required for targeting calls.