“Shall” isn’t plain English. . . But legal drafters use “shall” incessantly. They learn it by osmosis in law school, and the lesson is fortified in law practice.

Ask a drafter what “shall” means, and you’ll hear that it’s a mandatory word—opposed to the permissive “may”. Although this isn’t a lie, it’s a gross inaccuracy. . . Often, it’s true, “shall” is mandatory. . . Yet the word frequently bears other meanings—sometimes even masquerading as a synonym of “may”. . . In just about every jurisdiction, courts have held that “shall” can mean not just “must” and “may”, but also “will” and “is”. Increasingly, official drafting bodies are recognizing the problem. . .Many . . drafters have adopted the “shall-less” style. . . You should do the same.

Bryan Garner, Legal Writing in Plain English, 2001, pp 105-06.