Remove from Rig: Removes the blendshapes connection from the rig.

Apply to Rig: Connects the blendshapes to the rig.

Use Existing Correctives uses existing corrective blendshapes from a previous connection if there are any.

Remove Correctives removes the corrective blenshapes after the connection was made, leaving a lighter scene, but doesn't give the option to transfer the shapes to another rig.