My son has some biology coursework about osmosis in plants, so he can probably ignore all the interesting facts on Wikipedia about osmosis in fish ("Effect on cells").
Salt water fish lose water through osmosis, and need a mechanism to rehydrate and desalinate their cells. Meanwhile, freshwater fish gain water through osmosis, and need an entirely different mechanism to dehydrate their cells. So that appears to be two different systems with two different purposes.
But what if we look at a species of fish that thrives both in salt water and in freshwater, such as salmon. Presumably such fish need both mechanisms. The purpose now is not adding or subtracting but regulating. (Scientists call it osmoregulation.)
So we have different ways of thinking about the purpose of these mechanisms. If we are solely looking at freshwater fish, we see the purpose in a specific way; if we are looking at all kinds of fish, then we see the purpose of the mechanism in a much more general way. This relates to @seabird20's point that POSIWID should be plural.
This means that we have to be careful when using POSIWID as a simple systems thinking heuristic. We get different answers depending (if I may use a fishing metaphor here) on how widely we cast the net.