Did you mean, literally, less than 15 and not 0 or, more likely, greater than 0 and less than 15? Here are examples of both:
→ Not 0 and and less than 15. Range of -Infinity to 15, excluding 0.
<<if $Variable isnot 0 and $Variable lt 15>>
less than 15, excluding 0
<<elseif $Variable is 0>>
exactly 0
<</if>>
→ Greater than 0 and less than 15. Range of 1 to 14.
<<if $Variable gt 0 and $Variable lt 15>>
less than 15 and greater than 0
<<elseif $Variable is 0>>
exactly 0
<</if>>