When should a source be marked as available online? Short background: I'm a first year undergrad student and one of our study untis aims to educate us on academic reading and writing, including citation styles. Our Faculty requested we use the IEEE standard.

We have to gather the bibliographic data for some articles and books given to us. Two of these are articles (one is a journal article and one is a magazine article), each having a DOI and each having a PDF version available for purchase.

Would these two have to be marked as available online (with the doi/url) given or are there other conditions for these to be marked as availble online?

Better yet, to generalise the question, what are the conditions to have a source marked as available online (thereby also providing the DOI/URL of the source)?

By "marked online" I mean that the bibliography citation would have [Online] in the case of the IEEE standard
There are two general cases for supplying a URL or DOI for information:


Information where a URL is required in order to locate and uniquely identify a source, e.g., referencing a website or a software distribution. In this case, you must always include the link, typically along with the date retrieved, as the contents at the end of the link may vary over time.
Information where a DOI (or similar persistent link) is a "nice to have" that gives a fast link directly to the original source, e.g., referencing a standard journal article. In this case, it is "best practice" to always include the link, but not strictly necessary.


Personally, I am lazy, and include DOIs in the second case only when the bibliographic source I'm pulling from includes them in the reference already---unless specifically asked for them I will no go through the trouble of looking up what is (strictly speak) redundant information.  For this same reason, whenever I am hard up against a page limit, the DOIs are the first thing to be deleted, because it is better to delete redundant bibliographic information than to remove content from the text.