1) It means that you can transit via one (and one only) of the cities mentioned in the middle. Whether you can have a stopover at any of these depends on the rules on stopovers. This is rarely allowed on US domestic fares.
2) This is boiler plate text that is included in the fare rules for fares originating from many cities. It will be relevant for some and not for others. Similarly, routings sometimes allow transit through some cities even though there are no actual flights between the relevant points for the same reason.