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Locomotion (jump) > Blend Per Bone > Spine > Attack
Indicates the animation migration and in this case from the Locomotion state machine is in the (jump) state which is then migrated through the Blend Per Bone starting at the > Spine bone Using the > Attack sequence.
The resulting final output will be the player model Jumping while attacking at the same time.
If the migration is Locomotion (walk) > Blend Per Bone > Spine > Attack then the player model will Walk while Attacking.
Also referred to as animation layering or in the case of Motion Builder an over ride.
The migration order depends on what needs to blend first as part of the hierarchy so in most cases one would start first with Locomotion as the first layer always starts at the root. AKA where goes the legs so goes the player model. 
Next you could migrate through a Blend Per Bone to layer the Attract, or what ever you wish, using the Spine, or what ever you are using as the first joint of the upper torso, and will over ride the torso animation coming in from the locomotion state machine.
In general try avoiding the addition of an action state to a locomotion state with in the same state machine as it should only be used to make the player move.