The scale affects the ruler in Surveyor, so that you can make accurate measurements.
If you want to design a layout in Surveyor that will actually be build to scale, then you would select the correct scale so that you can see how things will turn out when you build.
Using the ruler, you could get measurements from Surveyor that you can use when building the actual layout.
If you are building a prototype RR or just don't care, then just use Real scale
Hope this was of some help
FW