On 1 Dec 2011, Anton Prowse raised a bunch of issues with the Multicol CR. One of which was:

Section 2 (The multi-column model) says:

Column boxes act as the containing block for their content.

Various specs use similar wording, but it's always clumsy. Obviously they only act as the containing block for descendants for whom they are the containing block. We could say something like "Column boxes participate in the tree of containing blocks", but there's currently no such concept described anywhere (despite existing conceptually), so instead I suggest we just say:
Column boxes are block container boxes.
Then the fact that they form part of the tree of containing blocks falls out naturally and doesn't need explicitly stating.

There was a minor edit made to the spec - email 9 Jan 2012. This seems to be the current wording.

Issue was returned to 15 April 2013 with a suggestion that the wording be revised again, as far as I can see that didn't happen and I'm posting this to see if anyone has an opinion on the matter now.

This is one of a list of legacy issues I'm digging out of the www-style archives.