I believe that there is no such thing as a falsetto. I think 'falsetto' is a word which itself is a construct to describe a legitimate kind of voice which for the past "while" has not been held in the high regard it deserves, excepting such masters as Frankie Valli. & Perhaps Lou Christie.
These things are hard because of the inconsistency of terminology between different people (even experienced singers and teachers) to describe physical and aural phenomena. I think the difference between the use of different registers or mechanisms is much harder to deal with in prose than if we could just demonstrate for each other what we're talking about.
I'm a high baritone of the sort various teachers have always argued over whether I should have really been a dramatic tenor, but definitely not the "light, high" tenor type.A good note for me to demonstrate different mechanisms is the E-flat above middle C because it's relatively easy to sing and make audible in multiple ways. I can sing it in pure chest. It's a big, bright sound. It takes some sheer physical effort on the part of the abdominal and intercostal muscles for me to produce the sheer volume of air to do it, but I wouldn't call it difficult by any means. Since that's fairly high (at least for me) to sing in pure chest, I feel a certain firmness (which I'd really not want to call tension) in the muscles that hold the throat open and that maintain the vowel. I would only sing this note this way if I wanted to make it loud and produce a dramatic effect. The physical sensations from the actual sound itself are mostly below the lower jaw - some in the throat and a great deal in the chest cavity. I can sing it in mixed voice. This is the way I'd sing this note most of the time. To do this, I have to engage the support muscles (which feels more like holding back air than producing air). It's getting just high enough, that I have to be aware of consciously relaxing the throat, since instinct would start to bring in tension at this point. The mixture of head and chest voice produces a somewhat darker vowel than pure chest (again, these concepts of "light" and "dark" are so much more easily demonstrated than described). One big difference between mixed voice and pure chest is that, in pure chest I feel I'm exerting more physical effort and that this effort would increase substantially if I had to continue up the scale without changing anything. In mixed voice, it's less a matter of effort than a balancing of forces and I feel well poised to either ascend or descend the scale. The physical sensations from this sound seem very centered in the mouth and nasal cavities, but there is also some feeling of "buzz" in the chest cavity.I can sing it in pure head voice. I would do this if a tender passage were marked "pp" and the musical passage wouldn't require swelling to full voice without an interrupting breath. For me, this is much an exception as pure chest on this note - something done rarely for a special effect. I feel considerable involvement of the support muscles and I have to keep the throat relaxed. The sound does, however, have carrying power and a certain amount of "shimmer" or vibrato. The physical sensations from this sound seem to start above the upper teeth and go up, with the majority of it feeling like it's around and behind the eye cavities.I can sing it in falsetto. This feels very different from head voice. There is no shimmer, no vibrato and my support muscles aren't engaged. It's very light, airy and detached. It wouldn't carry very far. There's no tension or temptation toward tension in the face or throat, but there's also no place this sound could "go." About the only time I'd actually use this in performing is if I were doing something that required a particular sound in an early music a capella choir that had to be kept relatively quiet. In my case, the physical sensations from the sound seem centered around the back of my head and the ears.There is yet another mechanism that I can use, but in my voice that E- flat above middle C is rather low for it to engage. This is what Jerome Hines and some others have called the "whistle register." Men rarely use it, but coloratura sopranos use it heavily as the basis for those "overdrive" notes like the staccati above high C. These are the notes that feel like they're coming out "the top of your head" because that's where the sensation of the sound is. You feel the support mechanism definitely engaged. Some singers can produce these sounds with an almost completely closed mouth, but at the same time you have to feel a really pronounced lift of the cheekbones and soft palate.Based on my own experience of imitating them, I think it's the use of this "whistle register" than can be developed quite significantly by some singers and reinforced by the support muscles, that produces the unusual piercing but sometimes musically effective sounds that have been termed "falsetto" when used by singers like Frankie Valli and Lou Christie.Other than the falsetto, which is rarely used in operatic performance, and the "whistle register" which is rarely used by men in operatic performance, I think all voice types make use of all the other positions or, if you will, mechanisms. Basses and baritones use them all, but still spend most of their time in mixed voice. The same with mezzos and contraltos, but even for them "pure chest" is a rarity. Frequently, what is called "chest voice" in referring to a mezzo or contralto is, in fact, her use of mixed voice. Tenors rarely use chest voice, spending most of their time in mixed voice and head voice. Sopranos and counter-tenors spend most of their time in head voice, generally using mixed voice only for dramatic effects in the lower and middle register and, on the other hand, using the whistle register for the extreme top notes (if they sing those at all). On exception to this was Callas who used mixed voice for her middle and lower registers, head voice for her upper middle and full voice high notes and her "whistle register" for the acuti. The use of mixed voice gave her middle and lower registers a very unusual and distinctive sound and is almost certainly what di Stefano was referring to when he made the comment that she "sang like a man."The soprano or mezzo who is said to have "no breaks" or a "remarkably even" voice is one who uses almost exclusively head voice and has the ability to color the head voice so as to produce sufficient brightness and power in the middle and lower registers. Callas' use of the mixed voice for middle notes, head voice for the upper middle and some high notes and that whistle register for top notes caused some to complain that she was technically faulty because of her "three register" approach to singing. Others consider that a strength of hers and that it gave her a more extended range and variety of color than other singers. Tebaldi, on the other hand, seems never to have used or developed the whistle register and, hence, was never known to sing anything above high C. Tebaldi did, however, use pure chest voice for some of her lower notes to produce dramatic effects in roles like Tosca and, later, Gioconda, which some people feel did damage to her instrument but which certainly produced striking effects.Fischer-Dieskau used head voice more than most baritones and took it lower than most baritones would try to do. He also rarely took chest voice as high as most baritones would do. My guess is that, were we to get a description from his ENT, we'd be told that he had the larynx of a tenor with the head cavities of a baritone.