[OS X TeX] Format declarations?

[david craig]

Can someone clarify for me what purpose exactly the format declarations 
at the beginning of a tex file serve?   And what are the recognized 
formats for these declarations?

Roughly, as I understand it, they tell the underlying engine (usually 
pdflatex these days, I guess) how to process the file in the absence of 
some other instruction.

What's the difference, though, between, say

%&latex

and

%!TEX latex

The latter syntax (which I copied from a TeXShop version history page) 
doesn't seem to have an effect.  (It still produces a pdf and not dvi.) 
Am I interpreting that page incorrectly?

<http://www.uoregon.edu/~koch/texshop/version.html>

I'd like to understand these declarations a bit better, but Google keeps 
filtering out the special characters so it's been hard to pin down.

As a follow on, the auto tex generation at arXiv.org won't process a 
file that begins with %&pdflatex.  (They're uppity that way -- says it's 
not a valid format.)  Is there any way to set up those format headers so 
TeXShop will run pdflatex and arXiv will run latex?  (I suspect the 
answer is to leave off the format header entirely.)

Thanks,
David Craig


<http://www.panix.com/~dac/>