It's a pretty tough one because in most cases either can be used. IMO "under" suggests closer proximity than "below".
Think of 'super' as over or above and 'sub' as under or below.
sub is a prefix that basically means below/beneath or under, as in subterranean (below ground), or subtitle ( a smaller title below the main title). A sub-camp is a smaller camp under the purview of a main camp (usually a concentration camp)
The site under Related Links below has a number of very good images.
refer to question below:
There really is no difference
the difference between 17 and under 17 years old is that under means younger than 17 and 17 is just 17.
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The main two branches of Islam are Sunnis and Shiites. Refer to related question below for the difference between the two.
-8 - -17 = 9
There's no difference their both under the Department of the Navy.
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The mantle is below the plates.