Thanks for your report.

Note about

Draw > Polyline > Delete Segments between Two Nodes [OB]

From the reference manual:

"After the segments are deleted, the two limiting nodes are connected by a straight line segment so the polyline is still one continuous sequence of line segments."

I will change the title of this tool to "Delete Nodes between Two Nodes" which seems more appropriate.

In the context of

Modify > Break out Segment [D2]

the entire 'piece' of the polyline is broken out that is limited by other entities or a self-intersection. A node is not a self-intersection.

Modify > Break out Manual [B2]

The order of the clicked breaking points is relevant in relationship with the polyline direction. This could be improved but needs more research into cases in which this behaviour might be required.

OB = intended behavior
OB remains different for last & second last node of a closed poly.

D2 = intended behavior

B2: Please investigate what is in the red clouds.
Also remark the 3 verticals in B2+keep in the upper trials.

CVH

"I will change the title of this tool to "Delete Nodes between Two Nodes" which seems more appropriate."

"segments" =about the same as "nodes".
Disambiguate "delete" with "remove".
The open space between the dots is confusing too.
Usually red = new in QCAD Icons. (Except where it is not)

CVH

"segments" =about the same as "nodes".

A segment is the line or arc between two nodes. Nodes are the points between two segments or at the start / end of the polyline. If two consecutive nodes are chosen for the tool, the segment between them is not removed, since there are no nodes between them.

Disambiguate "delete" with "remove".

Agreed.

Sorry my bad ...
"node" =about the same as "vertex".

Then "replace" if even better.

Replacing multiple segments between two non-consecutive nodes replaces it by one single connecting segment.
Replacing a single segment between two consecutive nodes replaces it by an identical segment.

CVH