Inside a Pose Asset you can specify if you want the animation to be Additive or non-additive in the form of Full Body.
Usually, for full body animation, you will want to leave this as non-additive. This creates 0 or 1 weight values (0.2 is the same as 1 for example).
Setting to Additive and providing a value between 0 and 1 might cause your character to compact, expand or scale undesirably.
However, for facial animation, enabling Additive will give you more precision between min/max influence of a given curve.
Above we set various weight values to control the amount of smile on the right side of the character's face.
You can also specify the Base Pose to use when converting to Additive.