Machine Between - calculates new Z Heights at the Stepdown specified (in the Stepdown frame). The Reference Toolpath Z Heights are then excluded from use. This leaves you with the effect of Machine Between, which can be useful when rest machining with the same size tool to try and reduce the terracing.
Re-machine and Machine Between - calculates new Z Heights at the Stepdown specified (in the Stepdown frame). In this case, the Reference Toolpath Z Heights are NOT excluded from use. This can be useful when rest machining with a different tool from the one used in the