The most basic requirement of service is to satisfy the guests. The low-level service is "the guest asked you to do it, and you did it", the intermediate service is "the guest didn't ask you to do it, but you did it for the guest", and the high-level service is "the guest didn't expect it, but you did it for the guest, and then the guest was pleasantly surprised to find that you knew what you wanted better than ta himself".

I hope my answer is helpful to you.