A NumberPose holds the value applied to a specific FACS control. The control which is affected depends on the name of the NumberPose.
NumberPoses are the building blocks of facial animation and, with Keyframes, make up KeyframeSequences.
NumberPoses and Facial Animation
Although a NumberPose is assigned to a single FACS control by name, that control can in turn affect multiple bones of the face rig. FACS controls act as an abstraction layer between facial muscle movements and how they are defined in the rig.
NumberPose and hierarchy
Contrary to Pose Instances, NumberPoses cannot be parented together. However, they all must be stored in a Folder named 'FaceControls' that is a child of the 'Head' Pose.
The Roblox animation system applies NumberPose.Values to the corresponding FACS controls. Because those controls only respond to values between 0 and 1, the values calculated by the animation system are always clamped to that range.
The value that will be applied to the FACS control corresponding with the NumberPose.