These roles, known as swing starters, are intended to provide more flexibility with pitching rotations when using pitchers across baseball eras. For instance, if you had a pitcher who threw 600 IP you could place him in the "Starter 1" role and use two 200 IP pitchers in the "Starter 2A" and "Starter 2B" roles. "Starter 1" would pitch every other day, and the "Starter 2A" and "Starter 2B" pitchers would pitch every 4th day. Regular starting pitchers are not allowed to be entered into the rotation after swing starters. Multiple pairs of swing starters are allowed in a rotation. Swing starters cannot be in the same rotation as Tandem starters.
There are two other combinations available (1,2A/2B,3A/3B OR 1,2,3A/3B) that provide even greater flexibility for mixing and matching the varied assortment of starting pitcher types (i.e. 100 IP, 200 IP, 300 IP, 400 IP, etc.)