Actually…you need to update your definition of Ampacity. There was an important change in it…
Ampacity. The maximum current, in amperes, that a conductor
can carry continuously under the conditions of use
without exceeding its temperature rating.
Ampacity is a board term that does apply to anything that has an current rating so, yes it applies to other things than just conductors of the wire type…bus bars are conductors as well, but I think you get the picture.
In terms of reporting the capacity of a system, be is service equipment or remote distribution equipment you have to understand what you are needing to report to the customer. In other words, if they are buying a house with a 200A service are they getting the potential capacity to handle 200A worth of loads (or 160A of loads continuously). You can report that as Current Capacity, Ampere Capacity, Current Rating, Ampere Rating or what ever your SOP calls for. The fact is you are simply validating the potential capacity with all “weak-link” factors figured in.
Now the problem I would have with your quoted statement is that a 200A current rated remote distribution panel (RDP) supplied by 4/0 CU (COU rated 230A @ 75) would have the potential ampacity (as defined) that would exceed the rating of the RDP. As long as the lugs can, within it’s listing, accept 4/0 CU then no harm is done, as long as the ratings of the RDP are supply side protected (OCPD) and as long as the calculated load is observed as not to violate the panelboard and enclosures listing then having a larger capacity conductor is technically an advantage to the reduction of I2R losses.
Panelboards have to be marked [408.58] by the manufacturer with the voltage and current rating so this is the starting point. However, this is only the maximum current rating of the panelboard and subsequently the enclosure also are not to be exceeded, ensuring it functions as tested and evaluated.
So maybe (and quite possibly so) I read your quoted statement incorrectly but based on how it is written it looks like you are saying a larger capacity feeder can’t be placed on a lower current rated panelboard and that is just incorrect.
Hope this helped…and not confused.