Additional Reference:
Clip studio puts these in a separate area called light table layers: https://www.clip-studio.com/site/gd_en/csp/userguide/csp_userguide/577_animationcell_plt/577_animationcell_plt_ope.htm

some additional conversation (IRC) and video from FlameReaper on how he uses it....
https://drive.google.com/file/d/1CVooALeUVgMDG9WgQd-zNVe1a7EptIjA/view

FlameReaper-PC
although this is without properly demonstrating how this would translate into either ToonBoom/OpenToonz/Clip Studio Paint use of the tools

FlameReaper-PC
but I would say what I did was more or less closer to how I would have done it in Clip Studio Paint

scottyp_win
ahh, good video.
FlameReaper-PC: is it always the last frame and previous frame that you need control over for the reference?
*last frame and next frame
FlameReaper-PC
in a given workflow you would usually receive a set of frames that you will need to redraw on your own

FlameReaper-PC
And using these frames you redrew, you will then use them to fill in the animation frames between them
So in that sequence I went to draw like this:

A-> B -> Middlemost frame between A and B (in this case: frame 2 out of 1, 2 ,3) -> either Frame 1 using A and frame 2 as point of reference or frame 3 using frame 2 and B as reference

scottyp_win
I think you did a good job explaining it in the video

FlameReaper-PC
Another reference video: https://www.youtube.com/watch?v=Z5mwgJr791A

scottyp_win
my question was with the UI. do you always use the frame directly before and after?

FlameReaper-PC
Not exactly before and after
It's basically start and end of the sequence, then I move on to fill in the blanks starting from the centers
so let's say there's a sequence like these:
① 2 3 4 5 6 ⑦

scottyp_win
keep explaining...but will be afk for a bit
be back in an hour-ish

FlameReaper-PC
I might be asleep after that :P
I'll leave my explanation here though

and if you have time do watch the link I left, that gives a good explanation on what the shift and trace feature Toonboom has can actually be used for :)

continuing... the circled 1 and 7 means they're the first and last frame for the given sequence, but you are presented with 5 frames to be drawn as inbetween animation for them. what would make it harder for you if the objects on the canvas are far from each other and they contain a lot of lines which change during the motion meaning you cannot simply

1. drag and trace one of the frames over

2. make a very rough drawing on what you think the next motion should be. In some cases this can help on a particularly difficult frame you cannot shift and trace on, but doing it means you take extra time to come up with a clean drawing which may or may not be consistent enough with the preceding and following frames

coming back to the sequence
I would begin by drawing frame 4 using frames 1 and 7 overlayed on top of each other and draw the new frame over them
while taking care to see where on the canvas should that new drawing be
After I've finished 4, I can go either way:

1. draw frames 2 and 5 first using frame 4 + frame ① in the case of the former and ⑦ in the case of the latter

FlameReaper-PC

2. draw frames 2 and 3 using 1 and 4 as reference, and follow up to finish by drawing frames 5 and 6 using frame 4 and 7 as reference

sometimes if there's a timing chart (examples: https://i.stack.imgur.com/GskAW.jpg) I may end up interpreting the sequence in a different ordering as per instruction. so if in the charts it makes it appear like frame 6 is the center of the motion that is being requested, then I will start with frame 6 and so on.

sometimes in a sequence there might be a partially drawn frame where you have to separate moving parts and non-moving parts to become two frames.
But the frame following the blank in-between you're about to draw contains a full body
So what to do in this case? = use the frames containing the moving and non-moving parts + the following frame as reference
Sometimes one might even go as far as two or three of the same reference frame (split into parts) to compose a full drawing to be drawn over as the shot transitions to the next keyframe

Based off the video in the previous conversation, the animator is putting the previous frame and the next frame to the new in-between position so the two drawings overlap. I added to the wireframe the ability to add 2 reference frames. I think they should default to before and after frames.

I am starting to think that these settings and positions for reference frames should not be saving to the KRA file since they are designed to be temporary anyway. When the current frame changes on the timeline, the reference position is wiped out. The only thing that needs to keep its state is probably whether to use reference images or not. When the animator wants to draw a new in-between they are going to have to move the drawings again anyway.

The wireframe alone is hard to tell how the workflow would be with this... I tried putting together a video with how it would work. I am not too good with video editing, so forgive the "popping" that happens with the onion skin docker.

This should help explain a little better a potential workflow for this.