In the book This is service design thinkingone One of the definitions quoting The Interaction Design Institute of Copenhagen is:
Service design is an emerging field focused on the creation of well-thought-out experiences using a combination of intangible and tangible media. It provides numerous benefits to the end-user experience when applied to sectors such as retail, banking, transportation and medical care.
The design of the service as a practice generally results in the design of systems and processes aimed at providing an integral service to the user.
Then, the same book spells the constituent fields. I have sorted them approximately according to the amount of overlap between these fields and those of UX:
- Interaction design
- Product design
- Ethnographic design
- Social design
- Graphic design
- Strategic management
- Chief of Operations
While it may argue that certain disciplines with UX could cover all of these fields, UXers hardly address strategic and operational management.
Another way to see this is that a retail business can hire a UX professional to improve their interactive products (internal and customer contact points), a business that aims to excel in the services front would normally approach the designers of services.
As others have mentioned, there is no agreement about what UX or SD includes or excludes, but the reality is that while they overlap, there are differences.
one Stickdorn, J.S.M., 1009. This is service design thinking: Fundamentals – Tools – Cases by Marc Stickdorn, Jakob Schneider (2014) Paperback, 1 edition. ed. Bis Publishers.