In order to make a digitally published document permanently available and referencable, independent of the concrete (and possibly changing) URL under which it is accessible on the internet, it is assigned a so-called Digital Object Identifier (DOI). DOI registration agencies or providers guarantee uniqueness and take care of resolving DOIs to the concrete documents on a technical level.
Dagstuhl Publishing registers a DOI for all works published on DROPS (the publication server of Schloss Dagstuhl) without exception and communicates this as the primary identifier (to authors, editors, but also indexing and archiving services).
In the first few years, the DOI service of another Leibniz institution, TIB, was used for DOI registration, itself a partner of the DataCite registration agency.
Since June 2020, Schloss Dagstuhl has now been a direct member of DataCite (see https://datacite.org/members.html), where it tries to represent the special interests of the conference-focused publication culture in computer science.