This is a relatively simple phase but it is incredibly important.
This is where a member of the campaign management team sits with a member of the marketing team and goes through the campaign targeting brief. Most people in the marketing department won’t be familiar with customer data (and nor should they). It is your job to make sure that you understand requirements and that everything is documented. If the brief has to change then this is where you take notes and make the necessary revisions. I used to enforce that this be done on a hard-copy but in the modern world an electronic copy with version control seems just as reasonable. Check all of the targeting and make suggestions for improvements if necessary.
It took me years to realise that if you want to properly understand marketing requirements you need to see what marketing actually plan to offer the customers. Ask to see a copy of the marketing offer (Letter copy, email template, banner ad, etc). You will learn a lot about who they are planning to target, what the message is and what the expected goal or outcome is. Does all of this new knowledge fit with the picture that you had built when you read the targeting brief? If not, ask for this extra information to be included. The targeting brief is not just for your immediate understanding, it is also an audit trail for anyone that needs to review the information in future (checking mistakes, repeat campaigns, similar campaigns, etc).