Last night I attended the STC Austin program "Four Candles - Just What is Simplified Technical English?", presented by Alan Porter of Quadralay and the 4J's Group. (Do watch the Four Candles video if you don't know this famous skit!)
Simplified Technical English is a writing standard created for aerospace/defense maintenance documentation, born of a deadly need for clarity (such as the worker who obediently "cut the power" with loppers and died). It's a controlled language because it restricts grammar, style, and vocabulary. Its goal is to stamp out ambiguity (one word = one meaning) and present technical complexity in the easiest language possible, to support users of diverse ages, abilities, and familiarity with English. If this sounds like the Plain Language movement to you, you're right: there's significant overlap. Boiling it down, Simplified Technical English has two parts: