However, the reverse view raises similar questions.  How do you know what runlevel 5 is?  (Hint: Someone with &quot;a passing knowledge of Unix&quot; knows that there&#x27;s no coherent definition of run level 5.  It&#x27;s &quot;power the system off&quot; on some Unices, for example.  Someone <i>without</i> a passing knowledge of Unix is probably at a complete loss as to what &quot;level 5&quot; could be.)<p>In actual fact, the multi-user boot versus single-user boot notion goes back to pretty much the beginning (<a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10207674" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10207674</a>), and the name &quot;multi-user&quot; is not exactly difficult to cope with if one has a Unix background.  It&#x27;s not novel or a systemd-ism.  It&#x27;s been a Solaris SMF milestone name for over a decade.  It was a concept used in the 4.1 Berkeley Distribution manual for init(8) in 1981.