What is the purpose of the IF statement below?

Northern Trail Outfitters (NTO) uses a numeric identifier for Subscriber Key. Customer data is stored in a data extension with the Subscriber Key set as a Primary Key.
Which step is required for NTO when creating relationships for this data extension in Data Designer'

A particular data extension need to be configured to store six months of data.
How should data retention be added to the data extension in Email Studio?

Northern Trail Outfitters uses a Send Log and sends more than one million emails per day. They want to execute daily reports on all subscriber activity without impacting send performance.
Which set of best practices should be implemented''

A developer wants to add an image to Content Builder via the API and retrieve the image's published URL.
Which method should the developer use?