I had a student once tell me proudly "I don't understand all this passaggio stuff.... I have no passaggio. what are they all on about?" I said "You have no passaggio because you haven't really found your upper register. What you think are high notes are probably transition or passaggio notes, if you handle them the right way."
Even if you support well, breathe well, have a loose throat etc. you still may have a few notes in your voice that require an extra rounding of the vocal chamber. Some teachers teach this through vowel modification. For myself, vowel modification just got me more stuck. The breakthrough for me was realizing that I didn't just have to round the vowel; I had to round myself. By this I mean that I had to learn to shape the throat with a deliberate roundness on certain upper middle notes in my voice.
Sometimes, if the context is right,this may just happen by itself; but if you try to sing a sustained step-wise passage finishing on an "ah" in the upper middle register, you will probably have to round the throat to experience any comfort.
The reason bel canto repertoire is so hard to master, is largely because of the composer's love of using passaggio notes, over and over again. If you don't work these notes out perfectly, you will never be happy singing Bellini or Donizetti. Of course, like my student the baritone (who was really a tenor), if you hide out in the wrong rep, you may never have any passaggio issues at all!