There is an error in darktable manual about retouch module, both in version 3.0 and in draft 3.4.
From manual 3.0
The top slider adjusts the merge scale feature. This setting allows to apply a common edit on multiple consecutive scales within a group starting from the highest scale (not including the residual image) down to the one selected by this slider. If the slider is set to 3 and the maximum scale is 5 then all edits that are added to scale 5 will be applied to scales 3 to 5. Edits added to scale 4 will be applied to scales 3 and 4, and edits added to scale 3 will be applied only to scale 3.
From draft for 3.4 manual
[…] edit to be applied to multiple consecutive scales […] starting from the coarsest scale […] down to the scale selected. [the example is the same of 3.0]
Now, in 3.2 (sorry for the very bad English):
For scale more fine that “merge from” value, the edit is applied only to edited scale.
For scale equal or more coarse that “merge from” value the edit is applied from edited scale up to the coarsest scale.
For example, if “merge from” is set to 3 and the maximum scale is 5 then all edits that are added to scale 5 will be applied only to scale to 5. Edits added to scale 4 will be applied to scales 4 and 5, and edits added to scale 3 will be applied from scale 3 to scale 5.
(see screenshots below: cloned the eye on level 4, it is visible on level 4 and 5, not in 3).