What does "flow forbid" status mean in dfprobe output? It's on some magma or water tiles, but I cannot make sense of it. Also a nitpicking, but dfprobe lists both liquids as "water", which may be slightly misleading.
It should mean that the liquid at that position isn't updated by the game. Combined with the equivalent flag of map blocks, it can be used to stop water from flowing, or make it flow, if it ever gets stuck (a particularly funny bug I only managed to trigger once).
I've done some testing, and it looks like flow_forbid is normally set on tiles with at least 4/7 water*, or with magma of any depth. I haven't seen any effect on flow; that seems to depend entirely on the block flags. It looks like it has more to do with pathfinding/marking dangerous terrain.
*I've occasionally seen it set with less, but not consistently; there's probably a delay or something to help pathfinding deal with rapid fluctuations.
Yes, I've seen it mostly on 4/7 or more too, but I've found two distinctive exceptions:
1) in my waterway connected to an artificial waterfall (so constantly on move, theoretically) there are tiles without that flag and with it, including 3/7.
2) in top layer of magma, where I removed most magma so all tiles are 2/7 or 3/7 they are all "flow forbid", despite fluctuating between 2/7 and 3/7. I have my pumps off (and had them off for months of game time), so it's natural fluctuation.
And about that water text thing I mean the text bolded in this excerpt from dfprobe output:
temperature1: 12000 U
temperature2: 12000 U
biome: 4
geolayer: 15
Layer material: 171 / DIORITE /
water: 3
flow forbid
biomestuffs: 0x1ec066c0