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Where fast is the "name" of the limit and the number after the name defines maximum number of tasks that can run simultaneously using this limit. That's why you do not need a trigger between tasks "t1" and "t2".
There is no need to change the tasks. The jobs are run in the order they appear in the suite definition. Be aware that command order may be used to modify the order after the suite definition has been defined.
The following is a sequence if the jobs run in the normal order
And if you manually now re-run the "t1" (you go over the limit!) 
Using integer limits is a bit different. In the following example we define limit disk to be 50 (megabytes) and task using 20 (megabytes) each. This means that only two of them can be running any given time.
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