Through experimentation (rather than documentation) I've figured out how to work with the new Documents tabs in Xcode 12. I'm not sure what I think of this yet, but here is how they work:
In the Xcode 12 Navigator, you can either single click on a file or double click on a file, to cause it to open as a Documents tab.
However, if you double-clicked to open a file in a Documents tab, then the next time you try this, for a different file that is not already represented in a Documents tab, it will open in a new Documents tab, rather than replacing the file in the currently selected tab.
To see how this works, try the following:
Open an Xcode project and close any Document tabs that may open by default in result of where you left off with your last session.
Now single click on a file in the Navigator - it will open in a Documents tab.
Now in the Navigator, randomly single click one file after another; you will see each successively replace the preceding in the same Documents tab.
Now in the Navigator, double click on a file to open it. It will also replace the last one you opened. But then the next file that you single click or double click, will open in a new Documents tab.
Also note that:
A. When you single click a file to open it, it's tab will be displayed with an italics font, while if you double click, it will be displayed with a normal font. Think of italics as representing the file as "temporary" and normal as representing the file as "static".
B. If a file was opened with a single click, and it is displayed with an italics font, if you double click on it's name in the Navigator, it will switch to a normal font, switching from "temporary" to "static" status.
C. I am not aware on the other hand, of how to switch a tab back from "static" to "temporary" - this may not be possible?
D. If you have multiple Document tabs open, and you have selected a particular "static" tab, then if you select a new file in the Navigator (that is not already represented in any tab), it will be opened in a new tab, immediately following the currently selected tab, rather than at the end of the tabs.
E. Is it possible to have multiple "temporary" tabs open? No it isn't. And let's say you have 2 "static" tabs open, and you select the first, then open a new "temporary" tab, so that you now have a "static", a "temporary", then another "static". Now if you select the last tab (the second "static"), and in the Navigator you click on another file to open a new "temporary" tab, not only will it appear last, but your preceding "temporary" tab will close.
F. If you tend to open files not by selecting them in the Navigator, but instead by using command+shift+o, at least on first glance, it appears that it is only possible to open files as "temporary".