[Tex/LaTex] How much space between number and unit

siunitxspacingtypographyunits

Javier Bezos (famous of titlsec, tensind, spanish … packages; and now maintainer of babel) wrote a nice book on typography and scientific notation in spanish.

He says (pag. 82) that the space between the number and the unit should be a thin space on displayed equations; but a word space on the text. I couldn't find this distinction in the SI documentation; and I have no access to the ISO 80000.

I think that siunitx puts a thin space always. (I made a small text and \showoutput shows the smae glue and boxed between the number and unit.
I look to the siunitx code but I can not say say if \SI checks if tis in a display equation or in a in-line one.

My questions:

  1. Is the advice given by Javier an international convention? or it is only applicable to the Spanish language?

  2. Would it be possible to by siunitx to automatically detect the kind of equation and apply different spacing?

EDIT 1

From Javier's answer I understand that the typography convention that he explained in the book will work as following

In the text I will write sentences like the following. 
The value of the parameter $a$ is $5\text{~}\mathrm{m}$, 
but on the other hand, $3\,\mathrm{km} + 500\,\mathrm{m}$ and 
probably $c = 3005\,\mathrm{m}$.

Finding a displayed equation with units should be possible:
\begin{equation}
c = 3000\,\mathrm{m} + 5\,\mathrm{m}
\end{equation}
And isolated magnitude as display equation can be the answer to an exercise.
The result is
\begin{equation}
\boxed{3005\text{ }\mathrm{m}}
\end{equation}

Best Answer

At a technical level, detecting whether \SI is used inside display math is already implemented for the detect-display-math option. Thus adding functionality to make a choice of spacing based on whether \SI is used inside display math or elsewhere is quite feasible. Presumably this would require splitting the number-unit-product into two parts, with an alias meaning that the existing option continues to work: as usual, suggestions for names would be welcome. My main concern with adding such an option would be that I've never seen this type of variability in printed material: I'd like to see an 'official' source for such an approach before adding it.

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