In Native terms, a "spirit" flute, is your personal totem flute.......taken to the "extreme". Not just the block, but the entire flute carries that specific totem theme.
Mine is the Great Horned Owl, although if I had found one in any form of owl, I would have been happy.
Mine has a Great Horned Owl, sitting on a branch, on a tree trunk, as the block, and crossed GHO feathers in carved relief on one side, and a GHO in flight, (a little bit ghostly), also in relief, on the other side. It also has some decorative bands, and inlays, including my name inlaid, in front of the nest.
My wife's is the wolf, with a standing wolf block, running wolves on the side, a wolf head at the back of the block (whose eyes seem to glow), and so-on.
These are special flutes, and only extremely creative wood carvers, as well as great flute makers, can make them. There are only a few flute makers out there, who can do both the art, and the tuning, well.
These flutes represent the connection to the natural world, in a time when all creatures were inter-related.
The totems are based on the special qualities of the individual, that make them who they are, and the explanations of the types of people associated with specific totems, can be found on-line, at various native web sites. (Just google Native American totems.)
These, of course can vary, as most plains peoples have a totally different relationship with owls, than woodsland peoples. (smile) That's because woodsland peoples had a lot more owls around, than plains peoples did.
Since I am part Cherokee, from the Eastern Band (Northeastern TN), the GHO is a good totem for me. Plains people might not use an owl totem at all, due to a different association with owls.
Which reminds me.....on the topic of mixed blood......the figures I have seen, say that the Cherokee (Tsalagi) are the largest of the Nations, with about 250,000 descendants, but only 15,000 of those, are full blood, today. The Navajo (Dine'), are the 2nd largest of the Nations, today, although I do not recall the numbers on them.
Spirit flutes are simply a flute that represents your connection to Mother Earth, and all her creatures, and the totem they represent, makes a statement about who you are, as the player.
Naturally, such flutes tend to be pricey, and you must find a maker that can make what you want, at a price that you can afford, so there may need to be trade-offs in the design of the flute, vs. the cost of making it. That is also why you will be unlikely to have more than 1 true "spirit flute"........and many people may not even know of it.....because you value it highly and seldom take it anywhere.
Mine is not my most expensive flute.......a drone from Geoffrey is my most expensive flute.....but it isn't far behind, in cost, and my wife's was approx. in the same price range. (Those budget constraints, you know.)