[OSM-dev] osmosis diff

[BG]

Hej.

I use osmosis because of the diff imports.
I do not have the complete planet in my database. only 3 countries.
While making a diff import, does osmosis only update my countries or 
does it change every data of the planet?
And as i know, there is a possibilty to use a boundingbox. Is it also 
possible to use more than one bbox?
This question makes only sense, if osmosis updates the whole planet.

thank you in advance.
Bernd