> "On-the-Fly Reindentation" also has difficulty of interpretation.
> Does there exist indentation that is "Not On-the-Fly"?  What would that
> do?

Yes, the behavior should be described clearly.
"On-the-fly" can mean different things.  What's
important is just what the behavior is.

A guess is that "on-the-fly" here means that
indentation happens in an automatic or partly
automatic way, as you edit/type normally.
E.g., hit `RET' to get a new line, and you also
get that new line indented.

"On the fly" means any old time, in particular,
while you are doing something else.

It can mean that while doing something else you
can hit a key to do something else, and then
continue with what you were doing.  Or it can
mean that something happens automatically
while you are doing something else.  My guess
is that the latter is what's meant here: hit
`RET' to enter text on a new line (main action),
and get also the extra action of indenting the
text on that new line.

Indenting that's not "on-the-fly"?

You can manually, explicitly indent a block of
text, using command `indent-rigidly', bound by
default to `C-x TAB'.  It indents lines that are
at least partially in the region.

See the doc string.  You can incrementally
increase and decrease the amount of indentation.
And a prefix arg indents to exactly that number
of columns.

There can be any number of kinds of indenting.
Some, like `indent-rigidly, can be initiated
manually - just to indent.  Others can be
initiated automatically, in combination with
some other activity or depending on the context.