Turning a Service Type on or off will not selectively make a given system no longer heard on your scanner. Service types are based on what the frequencies, or talkgroups on a trunked system, are used for. Law Enforcement Dispatch, Fire Dispatch, EMS, whatever. If you turn the Law Enforcement Dispatch service type off, you would no longer hear a local PD, or Sheriff's office assigning calls to units in the field. That will not eliminate a neighboring city's PD dispatch, while leaving your own department to be monitored.
In the Nationwide Systems menu, you are turning on, or off, categories of systems based on how they are used. If you turn off "Air Shows" ,n the Nationwide Systems menu, the scanner will not check frequencies commonly used at Airshows for activity. If you leave Railroads set as "On", the scanner will check frequencies commonly used by rail companies. Again, these are not specific systems, but instead are groups of frequencies commonly used in one of the listed categories. If you have a specific Nationwide 'system' turned on, the scanner will go through any frequencies commonly used in that classification.