What is a DOI?

Answer

DOI stands for Digital Object Identifier. It is a unique alphanumeric string that is assigned to a source.

While many styles, including APA, MLA, and Chicago, encourage or require the use of a DOI when available, not all citation styles require them.
Some styles may prioritize a URL or other form of electronic retrieval information if a DOI is not available, while others may not require any electronic retrieval information for certain types of sources, such as those found in academic databases.
It is important to use the style required by your instructor to determine how to write and format citations or references.