It’s all documented well, please have a look here. Especially:
When you use the “must run after” ordering rule you specify that taskB
must always run after taskA, whenever both taskA and taskB will be
run. This is expressed as taskB.mustRunAfter(taskA). The “should run
after” ordering rule is similar but less strict as it will be ignored
in two situations. Firstly if using that rule introduces an ordering
cycle. Secondly when using parallel execution and all dependencies of
a task have been satisfied apart from the “should run after” task,
then this task will be run regardless of whether its “should run
after” dependencies have been run or not. You should use “should run
after” where the ordering is helpful but not strictly required.
EDIT
I also encourage you to read this post.