Why it matters when you are writing
Underneath the surface variation there are only a few systems. Author–date styles such as APA and Harvard put a surname and year in the sentence, which suits fields where recency matters. Author–page styles such as MLA point to a location in a text, which suits fields that read closely. Notes–bibliography styles such as Chicago push citations into footnotes so the prose stays uninterrupted. Numeric styles such as IEEE and Vancouver use bracketed numbers, which keeps dense engineering and medical citation compact. Recognize the system and most of the details follow.
The choice is almost always made for you, and the place to look is specific: a journal’s author guidelines, a department handbook, or the assignment brief. Where nothing is stated, take the convention of the field you are writing into — psychology and education default to APA, literature and languages to MLA, history and much of the humanities to Chicago, electrical engineering and computing to IEEE.
What actually costs marks is inconsistency rather than the choice itself. Half a reference list in APA and half pulled from a database in whatever it exported is instantly visible to a marker, and it reads as carelessness about the rest of the work too. Pick the style at the start, apply it to the first entry, and make every later entry match that one.
