What is the difference between preferred standard form and classical standard form?

Standard form is the way scientists prefer to write numbers e.g. the number 30,000 written in standard form is 3 x 104. The preferred standard form is the way engineers prefer the number to be written, e.g. the number 30,000 written in preferred standard form is 30 x 103. Engineers like to see the number 10 raised to the power of multiples of 3. E.g. 10-12, 10-9,10-6,10-3, 103, 106, 109, 1012 etc.