Understanding the grid classes ( col-sm-# and col-lg-# ) in Bootstrap 3
[UPDATE BELOW]
I took another look at the docs and it appears I overlooked a section which talks specifically about this.
The answers to my questions:
Yes, they are meant to apply only to specific ranges, rather than everything above a certain width.
Yes, the classes are meant to be combined.
It appears that this is appropriate in certain cases but not others because the col-# classes are basically equivalent to col-xsm-# or, widths above 0px (all widths).
Other than reading the docs too quickly, I think I was confused because I came into Bootstrap 3 with a "Bootstrap 2 mentality". Specifically, I was using the (optional) responsive styles (bootstrap-responsive.css) in v2 and v3 is quite different (for the better IMO).
UPDATE for stable release:
This question was originally written when RC1 was out. They made some major changes in RC2 so for anyone reading this now, not everything mentioned above still applies.
As of when I'm currently writing this, the col-*-# classes DO seem to apply upwards. So for example, if you want an element to be 12 columns (full width) for phones, but two 6 columns (half page) for tablets and up, you would do something like this:
<div class="col-xs-12 col-sm-6"> ... //NO NEED FOR col-md-6 or col-lg-6
(They also added an additional xs break point after this question was written.)
Here you have a very good tutorial, that explains, how to use the new grid classes in Bootstrap 3.
It also covers mixins etc.