What does "Sub-Miniature" really mean?
It just means "of greatly reduced size", or "very small". link1
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Interestingly enough,the "sub" found in the name d-sub for connectors is also for subminitaure, because they were one of the smallest connectors of the time, even though they're considered huge today. So it is a relative term that seems to imply "smaller than we've built before".