The DOI number is a unique identifier that is given to articles and books, much like an ISBN number for a book. DOI stands for Document Object Identifier. With the DOI number, you can easily find that specific resource. Try searching for a DOI number in a library database or Google to see how it works.
Not every article has a DOI number, as it's an optional feature and the publisher must pay for the addition (but it's inexpensive). Many scholarly journals use DOI numbers though, so you will see them in your research. Whether or not a resource has a DOI number is not a measure of quality; anyone can create a DOI number.
Every article with a DOI number will display it on the first page of the article as well as on the article's publisher website. DOI numbers are listed in library databases. If there is one, it will be clearly available. If no number is showing, that article doesn't have one.
For example, use this DOI number to see one of the articles published in CTU's scholarly journal, The Pinnacle: https://doi.org/10.61643/c365272
