Two different things are drawn as a rectangle full of tabs, and telling them apart decides how the whole interface behaves.
A cell of a split is a position in a layout tree. It exists because a pane was divided. Close everything in it and it disappears, because the tree rebalances. A cell is not a thing you own; it is a consequence of an arrangement.
A group is a set of tabs that belongs together. It survives being emptied. It can be detached into its own operating-system window and dragged to another screen. It is the unit that a window is.
Why the distinction is not pedantic
Every question you ask the layout has a different answer depending on which one you meant. What happens when the last tab closes? Where does a dragged tab land? What does "detach" produce? What gets restored after a restart, and in what order?
Build on the wrong model and the symptoms are diffuse and maddening: windows that vanish when you did not ask, tabs that reappear in the wrong place after a restart, a detached window that loses its identity the moment it is emptied.
The rule
The group is the unit. A window is a group that has been pulled out.
Which means grouping is a first-class operation and detaching is a view of it, rather than detaching being the operation and grouping an accident of where things happened to land.
On a phone
None of this survives a narrow screen. Below roughly 768 pixels there is no room for two groups side by side, so the honest thing is to flatten them into a single tab strip rather than to render a tiling layout nobody can hit with a thumb.