ORIGINAL: Mad Russian
You know you can't walk away from that statement without clarifying it.
Well, some containers move around a lot, and are abstract: armies, corps, divisions. Others are unable to move, and are located at a specific point on the map: cities, forts. It is my recollection -- and again, it's been a few years since I've had to think about this -- that if you put one of the first types of container into one of the second types then supply will be "free." I could be wrong about this, but as I pointed out, it should be easy enough to test by looking at the supply report that one gets by clicking on the "Supply" tab.