The positions are determined by the template. For example if you give modules a position left. All modules with position left are pumped up by this piece of coding in your template and displayed in your HTML.
PHP Code:
<?php mosLoadModules ( 'left' ); ?>
In the backend you can easily see what positions are defined in your default template.
Site->Preview->Inline with positions
If you want more positions, you need to change the template.
A position must also known by Mambo.
Which are defined in Mambo, can be found here.
Site->Template Manager->Module Positions
You can also define your own positions.