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This the starting paragraph of section 14.1 PDG review (PDF) asserts:

The flavour of a given neutrino is Lorentz invariant.

What does this really mean? A neutrino of a given flavour $\alpha$, i.e., $\nu_\alpha$ is a spinor field, and unlike a scalar field, it's not Lorentz invariant. Does it mean something simpler?

Moreover, what is the point (or intention) of this statement at the place where it appears in the review?

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    $\begingroup$ There is no need to respond to this, because I am out of my depth if you do, but if you are looking for a simpler explanation: is flavour invariance not necessary if all observers are to describe the same particle? Please look at bit.ly/2s23aq1 As I say, if this is rubbish then please just ignore it and my apologies. $\endgroup$
    – user154420
    Commented Jun 28, 2017 at 16:38
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    $\begingroup$ @Countto10, it is nor rubbish at all: flavor, like chirality, should be the same in all frames (unlike helicity for massive particles!). This is reaffirmed in the review lest the inquiring mind jumped on the rest frame of the lightest mass eigenstate and wondered how flavor oscillations would appear in that frame. As the book says, with relativistically invariant phases, you can't go wrong. $\endgroup$
    – Cosmas Zachos
    Commented Jun 28, 2017 at 19:55
  • $\begingroup$ Lorentz transformations act trivially on the flavor subspace, that is, don't mix up neutrinos of different flavors. Its as simple as that. $\endgroup$
    – Prof. Legolasov
    Commented Jun 29, 2017 at 5:58

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It's not the neutrino (or lepton) field that is invariant (as you said, it is not). It is the number and type of leptons. That charge is actually Lorentz invariant, else you could have an electron turn into a muon when changing reference frame through a Lorentz transformation.

At least, that's the only interpretation of the sentence that makes sense to me.

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