A process is a set of tasks, notifications, or activities that are completed in a specific order. In Process Director, a process is modeled using either a Workflow or Process Timeline. So, the term "process" in the context of Process Director, is really a just a generic term for a Workflow or Process Timeline. Both are processes, and the term "process" can be used interchangeably for either Process Timelines or Workflows.
Both Workflows and Process Timelines can implement the Run Process task to initiate a second process. The second process will run parallel with the process that starts it. The process that implements the Run Process task is called the parent process. Similarly, the process that is started by the Run Process task is called the child process.
Any number of child processes can be started by a parent process, and synchronization allows a parent process to wait for the completion of a child process before proceeding. When a process is started, it can optionally contain a copy of all the parent process' references (e.g. document, eForms, etc.) or just a reference to them. Process Timelines can call Workflows as child processes, and vice versa.
So, let's discuss a scenario where a parent/child process would be necessary, and where the Run Process task might be useful.
Let's say that there is a specific process for obtaining approval from the organization's financial analysts, such as a requirement to obtain multiple approvals in a specific order. Then, let's assume that all financial approval processes, whether requests for travel, or capital spending, or expense reimbursement, must be approved by the financial analysts if they exceed a certain amount.
One way to implement this would be to encapsulate Financial Analyst approval into its own process by creating a Workflow or Process Timeline to model it.
Once you have done so, you can create your travel, or expenditure request processes separately. In each of these parent processes, you can use the Run Process task to call the financial analyst approval process as a child process if the amount exceeds the threshold requirement.
By creating processes in this modular fashion, you can re-use a process many times as a child of other processes, rather than repetitively modeling it multiple times in different processes.