NXC Version 1.2.1 r5
The sub keyword allows you to define a function that returns no data.
Functions that do not return any value are sometimes referred to as procedures or subroutines. The sub keyword is an alias for void. Both of these keywords can only be used when declaring or defining a function.
In NQC you used this keyword to define a true subroutine which could have no arguments and return no value. For the sake of C compatibility it is preferrable to use the void keyword if you want to define a function that does not return a value.