In reply to ABD_91:
It actually Table 20-4. That appears to be an oversight.
Section 3.14.3 Simulation time unit says
The global time precision, also called the simulation time unit, is the minimum of all the timeprecision statements, all the time precision arguments to timeunit declarations, and the smallest time precision argument of all the `timescale compiler directives in the design.
I think that is what is meant