IMHO there is no true rating system as to what is "low", "medium", or "high" flow.
But, we all can pretty much agree what this "flow" looks like in our aquariums.
"High flow" can be produced by a maxi jet 1200 as well as a Sequence Hammerhead. It all depends on how far away from the flow source the coral/invert is located. A coral 10 inches from a maxi jet 1200 could be considered in a "high flow" area, whereas a coral 3 feet away from a return powered by a hammerhead would be considered "high flow" as well.
To me, "low flow" is a very small amount of water movement in a particular area or around a particular coral. This would be demonstrated by very little sway or movement to the corals polyps and possibly the accumulation of detritus or sand sitting in that area.
"medium flow" would be significantly more flow than "low flow" in that coral polyps are frequently swaying a full range of motion with even significant movement of the coral body as well. There would be no stagnant detritus build up in this area. The force of the current is sufficient enough to feed the coral, but not strong enough to blast the coral and discourage polyp extension (talking softies and LPS here)
"high flow" would be considered enough current to aggressively move the coral in all ranges of motion. Strong sweeping motions of the polyps, branches, and main body of the coral would be happening consistantly. Certainly, no settled detritus or sand would be in this area.
I think the main thing to consider here is that corals, and anemones, really do best when placed in "random flow patterns". Too much linear flow has never been encouraged, whether it be low, medium, or high.
My neptheas, lemnalias, sinularias, toadstools, and anemones all look much more natural with medium to high 'random current' flow passing around them. I acheive this by my 12 random jets from my 4 way and 8 way oceansmotions units.
My LPS, bubbles, torches, hammers look better in my low to medium random flow areas with frequent but subtles movement to the polyps.
This is just my interpretation of what low, medium, and high flow is, but I think this is the general consensus more or less.
Oh, and yes, I would think that our anemones are quite fortunate that they can relocate themselves to more ideal conditions within our tanks in terms of light intensity and flow requirements. My roses ocassionally move as the surrounding corals grow larger and block either the light or the flow they were used to.
