A DOI is a unique and permanent string of alpha-numeric characters that identify specific digital content, such as articles. DOIs are created and maintained by the International DOI Foundation
DOIs are assigned when a source is published electronically, therefore some older sources may not have DOIs
DOIs can be assigned to articles, books, abstracts, conference papers, etc. (since publishers or content owners have to pay to register items, many scholarly articles may lack DOIs)
Cahill, M. A., & Gregory, A. E. (2011). Putting the Fun Back into Fluency Instruction. Reading Teacher, 65(2), 127–131. https://doi.org/10.1002/TRTR.01018