Currently Comfort Plus is being sold as an add on, once you book a ticket in the Main Cabin, you can choose to upgrade to the section either for free if you qualify based on your Medallion status, or you can pay to upgrade.
However, Delta is planning to launch Comfort Plus fares as its own cabin starting later this week. With this change, there will also be changes to how Medallions can get access to the cabin, making it more restrictive.
For more info
you can view this post as well as this post
about the policies for Medallions
How Comfort Plus changes affect Medallions:- Medallion members will continue to enjoy complimentary upgrades to Delta Comfort+ based on availability, after purchasing a Main Cabin ticket and opting in for a Delta Comfort+ upgrade.
- Diamond and Platinum Medallion members will be upgraded to an available Delta Comfort+ seat at the time of ticketing, while Gold and Silver Medallion members will be eligible to receive upgrades three days and one day prior to departure.
- Medallion members will be notified of their new Delta Comfort+ seat assignment via email and will have the option to change to another Delta Comfort+ seat, if available.
- Only one travel companion traveling in the same reservation will be eligible for complimentary access to Delta Comfort+ when traveling on the same flight as the Medallion Member and the companion must be a SkyMiles member.
- The clearance window will be based on the lowest SkyMiles tier level in the reservation.