I am aware of the pages you linked to and have on a few occasions tried to look over things. Having read quite a few rather large standers and APIs my impression the &quot;Portable Interfaces&quot; is rather dim. They are disjoint and unversioned notes not presentable portable interface standards. There are also various internal protocols and interfaces which are not documented from what I have gathered.<p>In short SystemD is an implementation. And as they say in there own documents there implementation is and never will be a portable implementation.<p>The &quot;Interface Stability Promise&quot; is referring to the unit configuration file format and the command line interface. This is not stability of the implementation c interfaces.<p>If you want to see a proper portable specification take a look at the Wayland Protocol Specification. There or perhaps POSIX.