sonicnick on "Custom XML Import Default Values, Required Fields"

Hi There, I'm trying to migrate custom CMS content with user comments into WP.
I have seen the XML format file example here:

http://google-blog-converters-appengine.googlecode.com/svn/trunk/samples/wordpress-sample.wxr

But don't fully understand how all of the values work. `
Is there a specification somewhere that explains what values are required, what can be left out etc to still be able to import content?
Specifically these meta values

<wp:postmeta>
<wp:meta_key>_edit_lock</wp:meta_key>
<wp:meta_value>1210966825</wp:meta_value>
</wp:postmeta>
<wp:postmeta>
<wp:meta_key>_edit_last</wp:meta_key>
<wp:meta_value>2575641</wp:meta_value>
</wp:postmeta>

And
<wp:comment_id>8</wp:comment_id> - Iif I leave blank will this be auto generated on insert?