"Music Upper / Lower Margin" (Style|General|Page): what for?

In the dlg box "Style | General | Page", how the "Music upper margin" and "Music lower margin" are intended to work? What the terms themselves do mean?

Referring to a score without lyrics, chord names and figured bass (which may introduce their own spacing issues) and with "Min system distance" set equal to "Max system distance" to avoid vertical justification complications, I observe the following:

1) If "Min./Max. system distance" are set to be less than "Music upper/lower margin", only the distance between the 1st and the 2nd system of each page obeys the system distance, the distances between other systems do not go below the "Music margin" values.

2) However, if in the case above there is a vert. frame at the top of the page, neither the distance between the frame and the 1st system or the distance between the 1st and the 2nd system obey the system distance values (do the frame counts as 1st system or not?)

3) If "Music upper margin" is reduced/increased, the top system distance from the top margin is reduced/increased accordingly

4) However, if there is a vert. frame at the top of the page, neither the 1st system position or the frame position are affected by the "Music upper margin"

So, it seems to me that:

A) "Music upper/lower margin" interact with system distance: are they (partially) a duplicate of system distance settings?

B) "Music upper/lower margin" interact with page margins: are they (partially) a duplicate of page margin settings?

C) Frames affect "Music upper/lower margin" effect (which might make sense, if frames are not considered to be 'music'); bur precisely how?

What are these two settings actually intended for?

Thanks,

M.