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+Configuration of af-main using systemd
+======================================
+
+Mechanism to start user sessions
+--------------------------------
+
+The mechanism to start a session for the user of **UID** is
+to start the service **afm-user-session@UID.service**.
+
+This has the effect of starting a session.
+
+To achieve that goal the first is to start the user session.
+This is done using the 2 systemd directives [1]:
+
+    User=%i
+    PAMName=afm-user-session
+
+The first tells what is the user. %i is replaced by the parameter
+of the service: UID. So the user is referenced here by its number.
+
+For this user, the PAM script **afm-user-session** is evaluated.
+It is implmented by the file */etc/pam.d/afm-user-session*.
+That script MUST refer to *pam_systemd.so* for opening the session
+with systemd. It often takes the form of a line of the form:
+
+    session     optional      pam_systemd.so
+
+that is directly or indirectly (through includes) activated by
+**afm-user-session**. [2] [3]
+
+The effect of starting a systemd user session is to start the
+user services and the most important one: dbus.
+
+When the user session is started, the service
+**afm-user-session@UID.service** enters its second phase:
+activation of the user session for the framework.
+
+This is achieved by activating the target **afm-user-session@.target**.
+But activating a *system* unit from a *user* session is a
+thing that has to be safe. This is done by the program
+**afm-user-session**. This program runs as rot (with the set-uid)
+and simply execute *systemctl --wait start afm-user-session@UID.target*.
+Where *UID* is the user id of the calling process.
+
+[1] https://www.freedesktop.org/software/systemd/man/systemd.exec.html
+[2] https://www.freedesktop.org/software/systemd/man/pam_systemd.html
+[3] https://linux.die.net/man/5/pam.conf