Different persistent identifier systems exist (DPC, 2017), for example the URN, Handle, PURL, ARK and DOI. Depending on the purpose, an object can be assigned one of these persistent identifiers. With the PID guide (Netwerk Digitaal Erfgoed, n.d.), you go through about 25 questions, after which a PID that seems best for your organisation and goals will appear.

DOIs are increasingly accepted as the persistent identifier of your choice when it comes to data citation. This is noticeable, among other things, because systems that offer other persistent identifiers are also going to offer DOIs. At first, Dataverse Network only offered Handle and then switched to DOIs. In addition to URNs, DANS now also offers DOIs. We will therefore zoom in on the DOI below. 

DANS, 4TU.ResearchData and SURF handle persistent identifiers differently. 4TU.ResearchData uses DOIs, DANS uses DOIs and URN:NBNs and SURF uses the Handle system. Datacite DOIs are suitable and intended for citing data, the URN:NBN is primarily aimed at identifying and is less used as a citation tool. Handle is an 'all purpose' PID system and is especially useful for assigning PIDs to large quantities of objects (Netwerk Digitaal Erfgoed, n.d.).    

PIDs at 4TU.Centre for Research Data

4TU.ResearchData registers DOIs via DataCite Netherlands. Within 4TU.ResearchData, all datasets that are provided with the required metadata have a DOI. They all have an UUID (Universally Unique IDentifier). A UUID consists of 36 characters (32 letters/numbers and 4 dashes) in the form of 8-4-4-4-12 characters. For example: uuid:32c53005-a4f2-447c-b231-6cdb7dcdd17f. The total number of possible unique UUIDs is so large that it is unlikely that two identical UUIDs will be created.

The DOIs of 44TU.ResearchData are prefixed with the URL of the data centre and have the UUID as a suffix, for example https://data.4tu.nl/repository/uuid:32c53005-a4f2-447c-b231-6cdb7dcdd17f. On the landing page of the dataset it says: 'please cite/link this dataset as doi:10.4121/uuid:32c53005-a4f2-447c-b231-6cdb7dcdd17f'. The code 4121 stands for 4TU.Centre for Research Data.

If you want to find a DOI, put dx.doi.org or doi.org in front of it. Then you will always come to the right place. You can also use  resolve a DOI. The resolver must also be kept for the long term, of course. This is done by the international DOI foundation. There are no concerns about maintaining the resolver: "It's too big to fail".

PIDs at DANS

At DANS, all datasets have two persistent identifiers: a DOI and a URN:NBN. Both are automatically assigned when the data manager approves and publishes a deposited dataset. The DOI can be used by researchers to refer permanently to the dataset. DANS has been using the URN:NBN persistent identifier for sustainable access to all the material in the archive for a long time. DANS manages the Dutch resolver for the URN:NBN (DANS, n.d.).

A DANS dataset gets two PIDs. This looks as follows:

Schöpfel, Dr. J. (University of Lille, GERiiCO laboratory) (2019): Data Papers as a New Form of Knowledge Organization in the Field of Research Data. DANS. https://doi.org/10.17026/dans-zk3-jkyb 
DOI: 10.17026/dans-zk3-jkyb
URN: urn:nbn:nl:ui:13-iy-02u8

A URN:NBN is structured as follows:

• URN as the identifier scheme;
• NBN as namespace for so-called National Bibliographic Numbers;
• NL:UI to indicate that these are identifiers that have been assigned within the Netherlands;
• A unique code for the dataset within DANS.

PIDs at SURF

SURF has two flavours of PIDs:

• For the SURF Data Archive (SURF, n.d.a.) SURF uses the Handle system (SURF, n.d.b.). The SURF Data Archive is suitable for storing larger quantities of data for a longer period of time. 
• For the SURF Data Repository (SURF. n.d.c.), SURF offers DOIs in addition to Handle. To see what this looks like, take a look at the metadata of this dataset by Ishiyama (2011).  

Researchers/research institutions can also register their data collections via SURF and make them accessible with the aid of PIDs (SURF. n.d.b.). 

It makes sense to not only make research data available but also software code, posters and other research output. Some examples: 

• Software code
  You can make software code citable by publishing the code from GitHub to Zenodo. GitHub has a DIY guide available  (GitHub, 2016). 
• Posters and other research output
  Posters and presentations are often shared on Figshare (n.d.) or Zenodo (n.d.a). Within Zenodo you can also create a community (Zenodo, n.d.b.) where you curate the collection of output with a group of people.  

Each upload gets its own PID in this way (both Figshare and Zenodo have the DOI as PID). For research output that does not automatically get a PID, this is an easy way to make that output findable, citable and more visible.