Let me try to explain it better.
Well, I believe that Freecad evolved from being able to edit a single file to something that is able to open multiples files in tabs.
However, FreeCad has these configurations called workbenches. They change the whole interface considering what the user is doing in a particular file. And I believe this is the issue.
Consider you are creating a file, then selects the Part Design Workbench, then add a new Body, then a new Sketch. Like I usually do. Now you are editing a Sketch, adding a circle, for example, but now you have to add a value to use as diameter constraint.
Then, you open a new file, with another part so you can remember the diameter from there. Then you double click the Sketch to see the value of this diameter constraint. What happens? Nothing, you can't Edit (or open) the Sketch because the Sketch is still open on the first tab/file.
You will have to close the previous Sketch Edition first. You can see the close button there, but It won't work since it belongs to another file. You will have to change the tab and then close the Sketch edition, then go back to the second file...
Well, this process is a way to long, and this is not what multiple files edition should be. The user should do whatever it needs on each tab/file. Also, each tab should remain in its current workbench.
For example, you are editing to fix/adjust a part with the Part Design, while in another tab you are adding multiples files together with the A2Plus plugin. Every time you change the tab, you will have to change the Workbench every time because Freecad UI is the same for all the opened files. This is a pain to work with.
Did you see my point? Do you have any idea on how to improve it?
I would be so glad to see these changes in the next release.