When in Additive mode, an animation layer “adds” its animation to layers that precede it in the stack and affect the same properties.
For example, if AnimLayer1 and AnimLayer2 both contain keyframes for a sphere’s Translation property, the resulting Translation value for the sphere is the sum of the Translation values on both layers.
Note: In MotionBuilder, animation set on an Additive layer is always calculated as Additive, no matter what type of property is animated. Unlike in Maya, MotionBuilder calculates all properties animated on Additive layers in Additive mode, including boolean properties (such as Visibility) and Enum type properties (such as Layer Accumulation).
See Setting the animation layer modes.