I'm switching from MacOs to Windows 10 and used there Stay to manage app windows on different 'spaces' on the same monitor. In your discussions it seems the focus of DF is handling different monitors. Is there a way I could use DF to save and restore the state of apps in different workspaces in Windows?

Ok, sorry, I got it. 'Spaces' = Virtual Desktops. I downloaded the trial and tested it. I followed the dock-undock guide https://www.displayfusion.com/Discussions/View/automatically-saving-and-restoring-window-positions-on-dockundock/?ID=5540c67a-5e2b-4f41-b88c-de4d9c2fa354. I had 6 instances/windows of Firefox spread across 2 Virtual Desktops on the same monitor. It only saved restored the position of two (2) windows out of 4 on one virtual desktop. You overwrite the profile by saving the other virtual desktop and it "forgets" the position of the windows on the previous desktop. But when you double click on the profile, all bounds / parameters are being saved correctly. I guess DF doesn't know how to find all the windows saved when handling multiple virtual desktops. Manual activation of the profiles doesn't help. I hope this helps other people save their time. I'm out.

Precisely. Position and size were correct, but not restored for all six windows.

I delayed my reply because I'm in the middle of migrating from one computer to another and had to reinstall DF to try this out. Well, yeah, that did the trick! Neat! Why is 'relevant windows' by default?? This is a great piece of software, it only looks very complicated - as everything Windows... . May every new user / customer get this right.

Yeah, I see where the issue is, after working some hours with different monitors. I've noticed the new Task Mgr has improved in Win10, but they killed the Windows tab. DF can only save the title of the file / browser tab active at one point in time. But if you browse away within the same window or switch the tab in your broswer / open an extra file on the app 'watched' by DF, it would not know what to do with it after moving to the other monitor.
The dock-undock guide was good, but doesn't always work. I'll try with key shortcuts as suggested in that thread.
BTW, I have a Surface Book 2, and the system adjusts the windows when you rotate the screen. Win10 must have an alternative new way to handle windows...