There are currently many ways of starting software when a user session starts.
This is an attempt to collect a list of pointers to piece the big picture together. It's partial and some parts might be imprecise or incorrect, but it's a start, and I'm happy to keep it updated if I receive corrections.
### x11-common
`man xsession`
* Started by the display manager for example, `/usr/share/lightdm/lightdm.conf.d/01_debian.conf` or `/etc/gdm3/Xsession`
* Debian specific
* Runs scripts in `/etc/X11/Xsession.d/`
*`/etc/X11/Xsession.d/40x11-common_xsessionrc` sources `~/.xsessionrc` which can do little more than set env vars, because it is run at the beginning of X session startup
* At the end, it starts the session manager (`gnome-session`, `xfce4-session`, and so on)
* Started by `pam_systemd`, so it might not have a DISPLAY variable set in the environment yet
* Manages units in:
*`/usr/lib/systemd/user/` where units provided by installed packages belong.
*`~/.local/share/systemd/user/` where units of packages that have been installed in the home directory belong.
*`/etc/systemd/user/` where system-wide user units are placed by the system administrator.
*`~/.config/systemd/user/` where the users put their own units.
* A trick to start a systemd user unit when the X session has been set up and the DISPLAY variable is available, is to call `systemctl start` from a `.desktop` autostart file.
* If `/etc/xdg/autostart/foo.desktop` and `~/.config/autostart/foo.desktop` exist then only the file `~/.config/autostart/foo.desktop` will be used because `~/.config/autostart/` is more important than `/etc/xdg/autostart/`
* Is there an ordering or is it all in parallel?
### Other startup notes
#### ~/.Xauthority
To connect to an X server, a client needs to send a token from `~/.Xauthority`, which proves that they can read the user's provate data.
`~/.Xauthority` contains a token generated by display manager and communicated to X at startup.
To view its contents, use `xauth -i -f ~/.Xauthority list`
