Here you have control over which animation is previewed in the viewport. Default will use the Animation tool's default settings. In most cases, this will cause the character to use the reference pose, except when an animation sequence is being viewed. Reference Pose will use the Skeletal Mesh's Reference Pose. Use Specific Animation will open an option to select a specific Animation Sequence associated with your skeletal mesh to preview. Live Link Preview Controller will use preview using Live Link settings.