Each article that is published in Frontiers in Science and Engineering is assigned a digital object identifier (DOI). DOIs are standardized digital identities used across all major scientific publishers. DOIs can be used to cite and link to electronic articles. A DOI is guaranteed to never change, so can be used as a persistent identifier to permanently link to an electronic article no matter where it is stored. So, when referring to an article you can either use the traditional reference information (name of publication, volume, issue, etc.), or use the DOI.
Read more about DOIs.