When to spell out or when the acronym itself is sufficient

Solution 1:

A "Definitions" section, like a glossary, can be an extremely useful resource for readers. It doesn't follow, however, that readers will (a) read and thoroughly master the content in such a section before proceeding with the rest of the document, or (b) appreciate having to page back multiple times through numerous intervening pages (if the document is long) to find the appropriate definition each time a term makes its debut in the main section of the document. Some portion of your readers may not be aware of the "Definitions" section at all, if they are reading only parts of the document or if they forget that they bypassed the "Definitions" section at the outset.

As Edwin Ashworth notes in his comment, different publishers have different rules for handling subsequent appearances of terms that appear initially in a "Definitions" section—and those rules tend not to be negotiable. Nevertheless, it makes sense to put yourself in the reader's shoes and judge whether the convenience of including a same-page definition when a term first arises in the main body of a document constitutes a significant benefit to the reader, notwithstanding the existence of the reference section.

It's a judgment call, of course, and it depends to a large extent on the length of the document, the complexity of the terminology you're using, and the level of familiarity that you can confidently expect a typical reader of your document to have with that terminology. But making the right call will amply repay the effort you put into analyzing the situation.