On 2020-04-27 00:05, Phillip Helbig (undress to reply) wrote:
Some journals require upright Greek letters for constants as opposed to
variables, for example $\upi$ when used to denote 3.14159... as opposed
to a variable ($\pi$ is sometimes used to denote parallax in astronomy,
for instance). (Some journals define \upi as "upright pi", \upi as
"upright i", and so on.)
....
Or is there a difference between mathematical constants and physical
constants?

The principle probably comes from pure mathematics, but even there,
historically, they have not been available, just as well certain styles,
such as bold italic, for the simple reason that it was expensive in led
typesetting to keep them. Some journals would though have them, and one
could mark up the manuscript to get the right one.

Unicode changed that by adding those as characters (code points), in
addition adding some styles that are not properly semantic, like sans
serif and monospace variations. (The sans serif style is used for
tensors by some engineers, but I have found no example in mathematics,
physics or computer science of that.)

[[Mod. note -- Checking a few general-relativity textbooks, I see that
Misner, Thorne, & Wheeler "Gravitation" (W.H. Freeman, 1973) uses sans
serif for tensors and differential forms, but none of the other books
I checked do this.
-- jt]]

So when those are available, one can experiment with adhering to this
principle, upright for constants and italic/slanted for variables.

In some cases it may not be immediately clear what to use: Unicode
unifies the upright style with the original language letters.

TeX, by contrast, translates them automatically to italic in math mode.
For Greek, it only has the slanted styles, and in addition not the
uppercase letters that look like the Latin.

Perhaps because standard (La)TeX provides Greek letters only in math
italic, upright Greek letters are less common than upright Latin
letters, even when used in the same way (labels, units, symbols which
are not variables).

So I switched typing these mathematical styles directly in the input
file, and the fastest way to do that, both to implement and use, I found
is to use text substitutions. Then the LaTeX unicode math package was
insufficient, so I switched to ConTeXt.

When writing for a specific journal, one usually has to follow the house
style. However, if there is no rule, I prefer to do what is generally
deemed to be correct. What is generally deemed to be correct here?

You will have to experiment with it a bit.