How new is #overview? The information is invaluable.
It was around as a source code patch during the 3.4.3 days.
Someone---presumably the original author--contributed it to
devteam and it was added to what eventually became 3.6.0,
sometime in the dozen years between 3.4.3 and 3.6.0.
Some levels receive an automatically generated annotation,
separate from interesting features you've encountered such
as altars and fountains. You can add a manually composed
annotation to the current level with the #annotate command.
That will replace any previous manual annotation but if there
is an automated one then #overview will show both. Like
re-naming monsters and objects, assigning a new annotation
of a single space will remove the old one.
During play, #overview only shows levels that it considers
to be "interesting", comprising any level with any annotation
or with discovered features or the furthest level reached in
each part of the dungeon or a level with extra stairs leading
to another part of the dungeon (if they've been discovered;
I don't recall if they have to have been traversed). During
end-of-game disclosure, all levels that were visited get shown
regardless of whether they were interesting.
In to-be-3.7, plain #overview still shows only interesting levels
but if you precede the command with the 'm' prefix, all the
levels you've visited will be presented in a menu and you can
pick one to assign an annotation to even though it isn't the
current level.