In writing*, N.B. is an abbreviation of the Latin phrase nota bene [NOE’-tah BAYE’-nay], which when translated into English means “note well.” It is used in formal writing to call the reader’s attention to a sentence or passage that accompanies it in the text. In most cases the sentence consists of a side comment or a word of advice or caution regarding the content that had just preceded it or immediately follows it. Its fully-expanded meaning is, “please note the following” or “pay attention here [or] to this.”

N.B. is used most frequently in formal essays, textbooks, journal articles, and in reference works, all of which are types of formal writing that are intended to convey factual information. The abbreviation is rather formal, so it is seldom used in fiction and all but never in everyday or personal writing. I have seen it used semi-ironically in a few instances in casual communication such as notes and even in a couple of electronic texts, but these occasions took place mainly in academic settings. It can also appear in business correspondence, legal correspondence, or in internal business memos and other contexts where the sender and recipient(s) are likely to know what it means. The description cited in the comment that accompanies this question appears to be an example of the use of N.B. in business or legal correspondence.

• N.B. is never used in verbal communication, either formal or informal

from https://www.quora.com/What-does-the-abbreviation-N-B-mean-and-why-do-people-use-it?share=1