What are the Rules of Timescale directive

Rules

-  'Timescale directive, like all compiler directives, affects all modules compiled after directive,  whether  in  same  file,  or  in  files  that are  compiled  separately,  until  next 'timescale or a 'resetall directive.

-  Precision unit must be less than or equal to the time unit.

-  Precision  for  a  simulation  run  is  the  smallest  of  all  precision  units  in  'timescale directives. All delays are rounded to the nearest precision unit.

$time task reports simulation time in terms of the reference time unit for module in which it's invoked.