Termination Wait activities are always related to a Process Creation activity. The associated Process Creation must be located previous to the Termination Wait activity in the process flow, and it must have the property Keep relationship with child process selected.
For more information about Process Creation activities, see Process Creation Activity.
The combination of the Process Creation and its corresponding Termination Wait activity is very similar to a Subflow activity. The advantage of using a Process Creation and a Termination Wait is that you can add multiple flow objects between them. The Process Engine executes this flow object and the subprocess associated to the Process Creation Activity simultaneously.