At Evidence Explained Elizabeth Shown Mills’s website, summarized:

Every time we use a source, we want to create a citation that helps us. We’re not trying to satisfy Miss Thistlebottom so she won’t give us a D-minus. We’re creating a citation that will give us the directions we need. Page numbers in books, file numbers for documents, distinctions between editions—or between versions imaged by multiple online providers  …  all of these are not roadside weeds to annoy us. They are mile markers that keep us headed in the right direction.