[myelin]

Not the case in Kate's usage, but there's also the precision thing: 1.65k means 1650 +/- 5, whereas 1650 or 1.650k means 1650 +/- 0.5.

[nickff]

Most engineers get used to using 3 significant figures, because the rest is usually noise; some will 4 'sig-figs' if the leading number is a 1.