Whether new or changed services that are wanted by active targets should be started. Additionally, stop obsolete services from the previous generation.
The alternatives are
suggest (or false) : Use a very simple shell script to print suggested systemctl commands to run. You will have to manually run those commands after the switch.
legacy (or true) : Use a Ruby script to, in a more robust fashion, determine the necessary changes and automatically run the systemctl commands.
sd-switch : Use sd-switch, a third party application, to perform the service updates. This tool offers more features while having a small closure size. Note, it requires a fully functional user D-Bus session. Once tested and deemed sufficiently robust, this will become the default.