My imagination says they add Resort, when it is near a beach, water (lake or ocean) or even ski slopes.
Spa is added when they have a seperate area with several massage or facial services. An example is Bangkok with two Marriotts. The Riverside BKK Marriott added Resort & Spa, when they added the Spa services. If I am recalling correctly. Resort (recreational facilities) & Spa is obvious the Hotel has both.

I have researched the European Hotel/Spa facilities connected to spring mineral waters throughout Europe with serious Spa services and treatments often with official Medical care. The Hotels serve excellent foods in addition for improved well-being. If you have never taken advantage of these services, I would highly recommend it even for a few day. My favorite is a Sports hotel in Austria with radioactive spring mineral waters from the Hofgastein area. Excellent massage and food with peaceful surroundings. The village has a Hotel school, and the students give perfect service in the dining rooms along with excellent cuisine. A few day prove to be very invigorating unlike anything in the U.S.

I don't think a Hotel should add SPA to their name if they do NOT have real healing natural spring mineral waters as you find throughout East Cental and Western Europe!
City water just does not do the job of natural Sping water with many healing minerals in it.

Seems most major US Hotel chains were busy adding Spa services to meet the marketing demands created by SPA magazine. Notice how each Hotel chain has it's own exclusive line of cosmetics or Spa skin care products for up sales or add on sales!

I like the Spa services when they have well trained technicians contracted by outside corporations like the cruise ships offer. I understand that there are special college degrees in England in Spa services. Or even the Eastern Europeans have better training of skin care, then most Americans can offer. Especially facials and advanced forms of massage like Shiatsu, Deep tissue, Acupressure, Sports massage and Thai!

Of course, if any Hotels chains have excellent advanced Spa services, I would like to know about them. There must be some exceptions for discerning guests/clients.