The dbus-daemon manual makes several references to "well-known" things. Examples:
On platforms other than Windows, unix-based transports (unix, systemd, launchd) are the default for both the well-known system bus and the well-known session bus
This is the default for the well-known system bus and for the well-known session bus.
Rules with the own or own_prefix attribute are checked when a connection attempts to own a well-known bus names.
I get the feeling "well-known" has a kind of legalistic meaning in the context of the manual but i cannot find any reference it.
The documentation for the pydbus library explains it as:
The bus name. This identifies which application you want to communicate with. You’ll usually identify applications by a well-known name, which is a dot-separated string starting with a reversed domain name, such as
org.freedesktop.NetworkManagerorcom.example.WordProcessor.
you can see these listed if you busctl e.g:
$ busctl list
NAME PID PROCESS USER CONNECTION UNIT SESSION DESCRIPTION
com.redhat.NewPrinterNotification 739 applet.py pi :1.31 session-3.scope 3 -
com.redhat.PrinterDriversInstaller 739 applet.py pi :1.31 session-3.scope 3 -
fi.epitest.hostap.WPASupplicant 439 wpa_supplicant root :1.4 wpa_supplicant.service - -
fi.w1.wpa_supplicant1 439 wpa_supplicant root :1.4 wpa_supplicant.service - -
org.bluez 25710 bluetoothd root :1.370 bluetooth.service - -
org.freedesktop.Avahi 443 avahi-daemon avahi :1.3 avahi-daemon.service - -
org.freedesktop.ColorManager - - - (activatable) - -
org.freedesktop.DBus 1 systemd root - init.scope - -
org.freedesktop.DisplayManager 532 lightdm root :1.6 lightdm.service - -
org.freedesktop.PackageKit - - - (activatable) - -
org.freedesktop.PolicyKit1 533 polkitd root :1.7 polkit.service - -
org.freedesktop.RealtimeKit1 742 rtkit-daemon root :1.24 rtkit-daemon.service - -
org.freedesktop.UDisks2 432 udisksd root :1.5 udisks2.service - -
org.freedesktop.hostname1 - - - (activatable) - -
org.freedesktop.locale1 - - - (activatable) - -
org.freedesktop.login1 423 systemd-logind root :1.2 systemd-logind.service - -
org.freedesktop.network1 - - - (activatable) - -
org.freedesktop.resolve1 - - - (activatable) - -
org.freedesktop.systemd1 1 systemd root :1.1 init.scope - -
org.freedesktop.timedate1 - - - (activatable) - -
org.freedesktop.timesync1 379 systemd-timesyn systemd-timesync :1.0 systemd-timesyncd.service - -
org.opensuse.CupsPkHelper.Mechanism - - - (activatable) - -