The system has two randr screens, DVI-I-1 and DP-2. A single one-row iconbox was initially defined (under fvwm2):

Style * IconSize 32 32 32 32
Style * IconBox 0 -40 -1 -1, IconGrid 40 40

This definition has been adapted for fvwm3:

Style * IconBox screen DVI-I-1 0 -40 -1 -1, IconGrid 40 40

The windows are iconized with a Mouse definition calling Iconify, and restored according to the function

DestroyFunc DeiconifyAndRearrange
AddToFunc DeiconifyAndRearrange
+ C Iconify off
+ C All (CurrentDesk, Iconic) PlaceAgain Icon

Regardless of the above configuration, windows on DP-2 are always initially iconized according to the defined geometry, but on DP-2.

Moreover, the iconbox might jump screens.
Given windows w1s1, w2s1 on screen 1 (DVI) and w3s2 on screen 2 (DP):

• iconizing w3s2 puts the icon on DP
• then iconizing w2s1 puts its icon on DVI
• restoring w2s1 puts it back on DVI, and brings the icon for w3s2 to the iconbox on DVI
• on the other hand, with w1s1 and w2s1 iconized on DVI and w3s2 iconized on DP, restoring w3s2 will puts it back up on DP2 and move the icons for w1s1 and w2s1 onto DP2 as well.
• across these jumps, the geometry of the iconbox is respected.

With the explicit placement of the iconbox using the IconBox screen argument, all icons should always be put on DVI-I-1 regardless of the original window's screen.

I suspect that the jumping around part has to do with the parameters to DeiconifyAndRearrange but I haven't been able to find the Screen-related parameter to fix it.