The *.desktop files are scanned in these applications directories in this order.
A user custom GUI menu entry can be created by adding a *.desktop file in the $HOME/.local/share/applications/ directory.
Similarly, if a *.desktop file is created in the autostart directory under these base directories, the specified program in the *.desktop file is executed automatically when the desktop environment is started. See Desktop Application Autostart Specification.
Also, the Freedesktop.org "Desktop Menu Specification" chapter
"C. Integrating your application in the menus"  documents how to add
menu items; it states:
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If an application is intended to be installed by an unprivileged user for exclusive use by that user only then $XDG_DATA_HOME should be used as value for datadir and $XDG_CONFIG_HOME should be used as value for sysconfdir. If $XDG_DATA_HOME is not set, the default value of $HOME/.local/share should be used for it. If $XDG_CONFIG_HOME is not set, the default value of $HOME/.config should be used for it.
Reading the above it seems perfectly legit to customize XDG_DATA_HOME
pointing that variable to any user directory I need to add custom GUIX
menu entries... and I let handle that menu entries to my guix profile:
do I miss something?
Should I also customize $XDG_CONFIG_HOME to point to a specific
directory different from $HOME/.config?
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>> Anyway to make the new installed *.desktop files appear in the menu,
>> I have to logout and login again: I've still not found a command (or
>> configuration) to update the menu, "xdg-desktop-menu forceupdate"
>> does not work.
> Should it? It might only honor XDG_DATA_DIRS and ignore XDG_DATA_HOME
> by accident.
AFAIU xdg-desktop-menu is supposed to also update user menus
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> Other than that, restarting your shell (if running on X) might be a
> more lightweight way of refreshing the menu.
Restarting the shell means restarting the desktop environment?
I know how to do it with i3 (reload config) but I don't know hot to do
it with LXDE (or mate, Gnome3, ecc.)
 as I already wrote, I don't use any desktop environment graphical
menu so I rarely experience this issues, but when trying to help other
users I still cannot find a deterministic solution to this class of