DOI is the Digital Object Identifier that uniquely identifies, and for ever, the online location of a document.
On this web, we use DOI a lot to refer to scientific texts (when they have one). We do so through the template {{doi}} For instance, {{doi|10.1002/pssc.200304064}} produces [DOI: 10.1002/pssc.200304064], allowing to unambiguously target one of my works with the exact same title "Spontaneous coherence buildup in a polariton laser" (the others being [DOI: 10.1103/PhysRevLett.93.016402] and [DOI: 10.1016/j.ssc.2004.06.049]. The works are different but, halas, given the task to describe the spontaneous buildup of coherence in polaritons lasers, I was then unimaginative at more precise variations on the theme regarding the titles.
The DOI is one of the rare referencing things that works well, is robust, convenient and efficient.
DOI also stands as "declaration of independence", such as the Declaration of Arbroath (the oldest such claim, from Scotland to England).