I need some help here.

I'm using org 8.3.4.

I am exporting subtrees to LaTeX.

So I configure as much as possible per subtree.

First, I can't find where is documented the difference between

:EXPORT_LATEX_HEADER:

and

:EXPORT_LATEX_HEADER+:

Clearly, I need to use the latter form with my configuration (the former doesn't output anything), but I'd like to know precisely where the behavior is described.

Second, when I use several lines with

they  are concatenated into a single line.

This is annoying because:

:PROPERTIES:

...

:END:

results in 

\usepackage{mathptmx} % rm & math  \usepackage[scaled=0.90]{helvet} % ss

which is obviously unintended.

Could it be possible to add a '\n' between those lines?

Thanks for any help / clarification

Fabrice