What is the default value of TRY-VSCROLL in `(previous-line &optional ARG TRY-VSCROLL)`?

Question

The documentation shown with Ctrl h k up states:

<up> runs the command previous-line (found in global-map), which is an
interactive byte-compiled Lisp function in ‘simple.el’.
It is bound to C-p and <up>.
(previous-line &optional ARG TRY-VSCROLL)
Move cursor vertically up ARG lines.
Interactively, vscroll tall lines if ‘auto-window-vscroll’ is enabled.
Non-interactively, use TRY-VSCROLL to control whether to vscroll tall
lines: if either ‘auto-window-vscroll’ or TRY-VSCROLL is nil, this
function will not vscroll.
ARG defaults to 1.
If there is no character in the target line exactly over the current column,
the cursor is positioned after the character in that line that spans this
column, or at the end of the line if it is not long enough.
If the variable ‘line-move-visual’ is non-nil, this command moves
by display lines.  Otherwise, it moves by buffer lines, without
taking variable-width characters or continued lines into account.
See M-x previous-logical-line for a command that always moves by buffer lines.
The command C-x C-n can be used to create
a semipermanent goal column for this command.
Then instead of trying to move exactly vertically (or as close as possible),
this command moves to the specified goal column (or as close as possible).
The goal column is stored in the variable ‘goal-column’, which is nil
when there is no goal column.  Note that setting ‘goal-column’
overrides ‘line-move-visual’ and causes this command to move by buffer
lines rather than by display lines.
This function is for interactive use only;
  in Lisp code use `forward-line' with negative argument instead.
  Probably introduced at or before Emacs version 29.1.

The usual behavior of &optional values is that their default value is nil, but the default value for ARG is stated to be 1.

What is then the default value of TRY-VSCROLL?

Answer 1

Didn't read all your comments. But the doc string says two things that are relevant: (1) ARG defaults to 1 and (2) this command is only for interactive use.

However, IMHO, it would be better if the doc said that interactively ARG is the numeric prefix argument. And that always defaults to 1, not nil.` IOW, when used interactively the cursor moves backward one line.

The doc is also less than ideal by saying, on the one hand, that the command should only be used interactively (that's a guideline), and on the other hand, providing info about the behavior defined by the arguments when the function is called from Lisp. That's all OK, but it isn't expressed as clearly as it could be, IMO.

When called from Lisp, all optional args default to nil, always.

According to this doc, when called from Lisp, non-nil argument TRY-VSCROLL means the function tries to "vscroll tall lines".

Unfortunately, this doc (as well as the doc for function line-move, which is what previous-line calls in this case to do the job) says nothing about what the verb "vscroll" means.

Not only that, but there's nothing in the Emacs manual about what "vscroll" means. The Elisp manual doesn't use it as a verb, as far as I can see, but searching for it and gathering bits of info here and there from the search hits, I get the impression that it has to do with scrolling the window vertically by pixels. (I may be wrong about that).

A guess is that the doc for this function (and for line-move) evolved over time to try to accommodate additions such as argument TRY-VSCROLL, and it was never rewritten well to clearly describe the behavior. Someone (TM) may have added TRY-SCROLL without rewriting the whole doc string accordingly. Just a guess.