Fixed timeframe campaigns have a fixed campaign start and fixed campaign end date. They include email and social media promotions.
Rolling campaigns are specifically for email promotions. They do NOT have a fixed start and end date. Rather you define the number of email promotions you want generated as well as the number of weeks to distribute these emails.
For rolling campaigns you can choose to have an accompanying social media campaign as well. You can select a social media start and end date as well as the velocity of social media posts to create.