I'm not sure, but I think it is more of a "regulated market/fixed price w/index adjustments based on inflation". So a price will have been set at some point in the past, and is adjusted up a few percent every year, depending on inflation (typically less than 2% I
think for Sweden).
To be clear, there is a "black market" of sorts wrt apartments in Sweden -- not sure how that works, but I think it is people illegally sub-letting at a higher price and/or owners taking a "bribe" on top of the regulated rent -- and it can be hard to get apartments in certain areas (high demand, too few new buildings) -- but my impression is that on the whole, it works much better than an unregulated marked, like the one we have in Norway.
Perhaps someone from Sweden can comment further.