Laziness is the Mother of Invention. The AC Point.
Good programmers know the value of "reusable" code. The RV-C protocol also re-uses ideas whenever possible. This is obvious in the way the committee used very few distinct data scales. By being ruthlessly consistent in the way amps, volts, and degrees are scaled, not only did the committee make the programmer's job easier, but they greatly reduced the chances for error.
But the best example of the committee's dedication to code economy is the AC Point. This is a collection of several different PGNs, all with exactly the same format. There are four distinct AC Point formats, and two AC Fault formats, which are applied identically to AC devices including generators, inverters, chargers, and transfer switches. GENERATOR_AC_STATUS_1 thus has exactly the same format as ATS_AC_STATUS_1, with only the PGN number itself and the Instance byte being different.
Obviously this greatly simplifies the job of writing the code to parse this data, and reduces the chances of a documentation or coding error. But there are a few tricky points. Some devices are purely 120VAC - and report a single "leg" or "line". But some devices handle power that may be 120VAC or 240VAC "split-phase". Shore power, in particular, may be 30A or 50A. In the former case, whereas the power may be distributed to two physical lines, the lines are in the same phase and the total capacity is 30A. In the latter case, the two lines are 180 degrees out of phase, and each separate line has a capacity of 50A (for a total of 100A).
RV-C identifies the phase relationship using the "Complementary Leg" and "Phase Status" fields. In a split-phase scenario, the first field identifies the Instance of the other leg (it must be of the same device type, of course). The Phase Status confirms whether the two legs are in single- or split-phase.
RV-C does not have any general mechanism for associating outputs from one device to inputs for another. "Leg 1" from the generator may or may not be "Leg 1" at the transfer switch - it would likely depend on the installation.
It might be asked, if the AC Point was such a good idea, why isn't there a DC Point. DC is substantially different from AC in one key regard. An AC line always has one specific source. A DC line may have multiple sources. For example, the same DC bus may be fed by a set of batteries, solar panels, a charger, and an alternator, all at the same time. Characterizing DC power is substantially different than AC power, and thus is served by a very different scheme under RV-C.