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Engineering notation is scientific notation in which the powers of ten are limited to those where the exponent is a multiple of three, i.e, the powers are powers of 1000 (but written as e.g. 106, not 10002).
As an alternative for writing powers of 10, SI prefixes can be used. These also provides steps of a factor 1000, except if the unit has an exponent: in that case the SI prefixes provide only steps of a factor one million, one billion, or more.
A disadvantage both of using an SI prefix and of using engineering notation, compared with scientific notation, is that e.g. 500 µm and 500 × 10−6 m do not show the accuracy distinction between 5 × 10−4, 5.0 × 10−4, and 5.00 × 10−4 m. That can be solved by not using a range 1–1000 for the number in front of the power, but a number in the range 0.01–10. Depending on the case this may be suitable, or impractical. In the example we can use 0.5, 0.50, and 0.500 mm.
Another example: when the speed of light is expressed as 3.00 × 108 m/s or 3.00 × 105 km/s then it is clear that it is between 299 500 and 300 500 km/s, but when using 300 × 106 m/s, or 300 × 103 km/s, 300 000 km/s, or the unusual but short 300 Mm/s, this is not clear. A possibility is using 0.300 Gm/s, convenient to write, but somewhat impractical in understanding (writing something large as a fraction of something even larger; in a context of larger numbers expressed in the same unit this could be convenient, but that is not applicable here).
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