I think it's a feature of some tilling window managers that allow to put each virtual desktop assigned to each different monitors. So instead of the normal thing where when connecting an external monitor it extends the area of the virtual desktop, you could have the virtual 1 on the laptop monitor and the external with the desktop 2, so you could easily switch desktops that could have different windows, for example changing laptop monitor to virtual desktop 3 and keep external monitor with desktop 2.
I've never used it, but after hearing about this for the first time some time ago it made so much fucking sense compared to the chaotic way that normal window managers behave that I really want to experiment with it.
Yeah - this is what I meant. One virtual desktop per physical monitor.
I used to have it on a forked version of Openbox, but then I lost the binary and it doesn't compile anymore. If just makes much more sense to me and makes window management just a little easier. Especially when apps get confused about which monitor they're supposed to start on or try to be on both in an unhelpful way.