# What is the meaning of negative exponents in SI units?

Please could someone explain the reason for using negative superscripts in SI units, and more precisely when to use negative versus positive?

I do not understand, for example, why the volume of a fuel would be given in m3 but heat of combustion would be given in J m-3.

Negative exponents means per. In countries that were heavily influenced by the British, units such as meters per second would most likely be written $$m/s$$. In Europe and elsewhere it could be written with negative exponents as $$m.s^{-1}$$.
In your example $$J m^{-3}$$ means joules per cubic meter.