Suppose one task must be completed before another task can begin. In that case you would make the first task a "dependency" for the second. The second task will have a status of Pending until the dependency task is marked Done or Canceled.

The option to add a dependency is in the task details after the status. You can select any task in that client account as the precedent. But you cannot select a task from another portal.

You can even create a chain of dependencies, such that D begins when C is done, and C begins when B is done, and B begins when A is done.

Dependencies can be preserved in a template up to two levels. That is if you have Tasks A > B > C, and you make Task A a template, you will have the option to include Tasks B, but not Tasks C, in the template.