SUB is for subscripts
sub
The "sub" element is for treating a character as a subscript, displaying it in a smaller text than its surrounding text, and lowering its typographic baseline.
Examples
decimal 1000dec 1000<<sub dec>>
hexadecimal 400hex 400<<sub hex>>
binary 10000000000bin 10000000000<<sub bin>>
glucose C6H12O6 C<<sub 6>>H<<sub 12>>O<<sub 6>>
one-thousandth 1/1000 <<sup 1>>/<<sub 1000>>
care of c/o <<sup c>>/<<sub o>>