In many references including Wikipedia, electric flux and magnetic flux is written as $\Phi_{E}$ and $\Phi_{B}$, respectively. But they are not vectors (even then, they have to be bold) nor units. I think they should be written as $\it{\Phi}_{E}$ and $\it{\Phi}_{B}$. Shouldn’t they?

  • $\begingroup$ @BioPhysicist I disagree that this is opinion-based. This is a question on historical consensus. (It could be migrated to the History of Science SE, possibly, but shouldn't be closed as opinion-based.) $\endgroup$ – Steeven Sep 30 at 14:59
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    $\begingroup$ @BioPhysicist "I have never seen anyone make an explicitly stated distinction between italics and non-italics variables" $-$ you absolutely have, starting with this very site (though it is understandable if you have failed to notice). Notating the distinction between variables and operators by typesetting them in italics and upright type, resp., is essentially universal in professional typesetting. (Consider e.g. the difference between $sin(x)$ and $\sin(x)$.) OP's concern ("why does this universal convention not extend to this symbol?") is perfectly reasonable. $\endgroup$ – Emilio Pisanty Sep 30 at 15:15
  • $\begingroup$ @BioPhysicist As I said, if you haven't seen the distinctions (if you want, seen them explicitly stated), it is only because you have never looked at the natural places where they are stated. This is a perfectly objective question, and it isn't really covered in the off-topic categories in the canonical Meta thread on notation questions, either. Just sayin'. $\endgroup$ – Emilio Pisanty Sep 30 at 15:32
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    $\begingroup$ @EmilioPisanty I think this question falls squarely under Qmechanic's type 2: "What is the standard notation for this quantity?" Why do you think it's not covered? Because typography is distinct from a narrow idea of "notation"? $\endgroup$ – ACuriousMind Sep 30 at 16:26
  • $\begingroup$ @Acu I disagree with that characterization but I won't push the point either. $\endgroup$ – Emilio Pisanty Sep 30 at 16:59

It is generally very unusual for uppercase Greek letters to be typeset in italics, though this depends on where you are.

Quoting from the Wikipedia page Typographical conventions in mathematical formulae, as far as 'international' recommendations (specifically IUPAC, NIST and ISO) go,

anything that represents a variable (for example, h for a patient's height) should be set in italic type, and everything else should be set in roman type. This applies equally to characters from the Latin/English alphabet (a, b, ..., z, A, B, ..., Z) as to letters from any other alphabet, most notably Greek (α, β, γ, ..., ω, Α, Β, Γ, ..., Ω).

That same Wikipedia page blames the exception on the "American" system:

the American rule that italic type is used for all letters representing variables and parameters except uppercase Greek letters, which are in upright type.

In practice, this "American" convention is the default setting on LaTeX systems (unsurprising, since Donald Knuth is American, and the default mathematics package is the AMS standard), which has since become the de facto standard worldwide. Indeed, that is what this site does: note how italics are used on $a A \mu \Gamma$, which produces $a\,A\,\mu\,\Gamma$, with an upright $\Gamma$.

As a general rule, there are no hard rules when it comes to typesetting: there are only conventions, with varying degrees of universality.

If you are writing for publication, the only rules you have to follow is the style guide of the publication venue you are writing for. (As an example, if you are publishing in an APS journal, you have to follow the Physical Review Style and Notation Guide, which specifies in §IV.A.1, p. 16, that variables in Latin script are typeset in italics and variables in the Greek alphabet are typeset upright.)

If you are writing for your own use or for 'self-publication' (say, handing out notes to students), you can use whatever conventions you feel are most appropriate. There is definitely a strong push to use a consistent notation within each document (or series of documents), but that's about it.

If you are reading a document: unless the document's notation is so chaotic that it induces actual confusion about what the content is, complaining about superficial matters is... well, superficial.

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