A treatment is comprised of a feed-wide design or group of product set designs.
Single Treatment Feed
A Single Treatment Feed will replace the main image link of your feed with the new treatment. Linking a single treatment feed to your catalog in Commerce Manager will push the treatment to all ads accessing that catalog.
Single Treatment feeds are the easiest option to manage as any published updates to the treatment will automatically be updated across all campaigns without having to manually adjust anything on the ad level.
Multi Treatment Feed
A Multi Treatment Feed will retain the main image of your feed and add the treatments as additional images within your catalog. Creating a Multi Treatment feed allows you to assign specific treatments on the ad level to any campaign you'd like, without affecting the other campaigns.
Multi Treatment feeds provide you with more control over your viewers ad experience. Once a treatment is assigned to an ad, you can switch a design out in the treatment, without editing the ad again.
Because both Slideshow in Facebook and the Multi Treatment feed use your feed's additional images, if Slideshow is turned on for an ad, it will end up showing all of the added treatments for the feed which can lead to undesirable results