Argument used to specify the unit of a measure. The unit should always be specified in English singular form. Prefixes may be used. Engines will do their best to pronounce them correctly in the language used. Engines are expected to at least support the most common ones like "meter", "second", "degree celsius" and "degree fahrenheit" with some common prefixes like "milli" and "kilo". Can be used with TtsSpan.TypeMeasure.