I see i have been the most active asker on the forum now. But my english is far from fluent now (but it will) so lots of questions arise.

I read a book about the legal system of the USA. And I noticed that sometimes the author uses terms “suit”, “plaintiff” and “defendant” with articles and sometimes without articles. Here are some examples:

With articles: 1) A plaintiff is a party, and so is a defendant. 2) In general, this is done by filing a suit for foreclosure in court and obtaining a court order to have the sheriff sell the property at auction.
Without articles: 1) It sometimes happens that defendant already knows that plaintiff is filing suit and is willing to waive formal service of process. 2) Plaintiffs filing suit against administrative agencies must first exhaust their administrative remedies.

I have tried to analyse the difference but failed. So I am wonder if some of you could clarify this.